Explanation
In the Dockerfile context, The RUN instruction will execute any commands in a new layer on top of the current image and commit the results.
Layering RUN instructions and generating commits conforms to the core concepts of Docker where commits are cheap and containers can be created from any point in an image's history, much like source control.
https://docs.docker.com/engine/reference/builder/#run
Explanation
There can only be one CMD instruction in a Dockerfile. If you list more than one CMD then only the last CMD will take effect.
The main purpose of a CMD is to provide defaults for an executing container.
[https://docs.docker.com/engine/reference/builder/#cmd]{.ul}
Explanation
To tag the image into multiple repositories after the build, add multiple -t parameters when you run the build command:
$ docker build -t myrepo/myapp:1.0.2 -t myrepo/myapp:latest .
https://docs.docker.com/engine/reference/builder/#run#usage
Explanation
Note: Every shell command returns a status code
The HEALTHCHECK instruction tells Docker how to test a container to check that it is still working.
The HEALTHCHECK instruction has two forms:
· HEALTHCHECK [OPTIONS] CMD command (check container health by running a command inside the container)
· HEALTHCHECK NONE (disable any healthcheck inherited from the base image)
When a container has a healthcheck specified, it has a health status in addition to its normal status.
The command after the CMD keyword can be either a shell command (e.g. HEALTHCHECK CMD /bin/check-running) or an exec array (as with other Dockerfile commands; see e.g. ENTRYPOINT for details).
[https://docs.docker.com/engine/reference/builder/#healthcheck]{.ul}
Explanation
To remove one or more images use:
docker rmi [OPTIONS] IMAGE [IMAGE...]
You cannot remove an image of a running container unless you use the --force or -f option.
[https://docs.docker.com/engine/reference/commandline/rmi/]{.ul}
This removes (and un-tags) one or more images from the host node. If an image has multiple tags, using this command with the tag as a parameter only removes the tag. If the tag is the only one for the image, both the image and the tag are removed.
This does not remove images from a registry.
Explanation
The docker build command builds an image from a Dockerfile and a context.
The build's context is the set of files at a specified location PATH or URL. The PATH is a directory on your local filesystem. The URL is a Git repository location.
A context is processed recursively. So, a PATH includes any subdirectories and the URL includes the repository and its submodules.
This example shows a build command that uses the current directory as context:
$ docker build .
Sending build context to Docker daemon 6.51 MB
...
You use the -f flag with docker build to point to a Dockerfile anywhere in your file system.
$ docker build -f /path/to/a/Dockerfile .
The build is run by the Docker daemon. The first thing a build process does is send the entire context (recursively) to the daemon.
In most cases, it's best to start with an empty directory as context and keep your Dockerfile in that directory. Add only the files needed for building the Dockerfile.
[https://docs.docker.com/engine/reference/builder/]{.ul}
Explanation
Pull an image or a repository from a registry
docker image pull
Or
docker pull [OPTIONS] NAME[:TAG|@DIGEST]
[https://docs.docker.com/engine/reference/commandline/pull/]{.ul}
Explanation
Images and containers
Fundamentally, a container is nothing but a running process, with some added encapsulation features applied to it in order to keep it isolated from the host and from other containers.
An image includes everything needed to run an application - the code or binary, runtimes, dependencies, and any other filesystem objects required.
[https://docs.docker.com/get-started/#images-and-containers]{.ul}
Explanation
EXPOSE <port> [<port>/<protocol>...]
The EXPOSE instruction informs Docker that the container listens on the specified network ports at runtime. You can specify whether the port listens on TCP or UDP, and the default is TCP if the protocol is not specified.
The EXPOSE instruction does not actually publish the port. It functions as a type of documentation between the person who builds the image and the person who runs the container, about which ports are intended to be published. To actually publish the port when running the container, use the -p flag on docker run to publish and map one or more ports, or the -P flag to publish all exposed ports and map them to high-order ports.
https://docs.docker.com/engine/reference/builder/#expose
Explanation
You can specify a repository and tag at which to save the new image if the build succeeds:
$ docker build -t myrepo/myapp .
The output could look like the following:
To tag the image into multiple repositories after the build, add multiple -t parameters when you run the build command:
$ docker build -t shykes/myapp:1.0.2 -t shykes/myapp:latest .
https://docs.docker.com/engine/reference/builder/#run#usage
Explanation
By default, when you push an image to DTR, the Docker CLI client doesn't sign the image.
You can configure the Docker CLI client to sign the images you push to DTR. This allows whoever pulls your image to validate if they are getting the image you created, or a forged one.
To sign an image you can run:
export DOCKER_CONTENT_TRUST=1
Explanation
To show untagged images, or dangling, use:
docker images --filter "dangling=true"
This will display untagged images that are the leaves of the images tree (not intermediary layers). These images occur when a new build of an image takes the repo:tag away from the image ID, leaving it as <none>:<none> or untagged. A warning will be issued if trying to remove an image when a container is presently using it. By having this flag it allows for batch cleanup.
[https://docs.docker.com/engine/reference/commandline/images/#filtering]{.ul}
Explanation
To save one or more images to a tar archive (streamed to STDOUT by default) use:
docker save [OPTIONS] IMAGE [IMAGE...]
The achieve can be distributed through different channels such as: central file server, version-control system, sent it to you over email or shared it via flash drive.
[https://docs.docker.com/engine/reference/commandline/save/]{.ul}
Docker provides a command to load images into Docker from a file. With this tool, you can load images that you acquired through other channels.
[https://docs.docker.com/engine/reference/commandline/image_load/]{.ul}
Explanation
To create a tag TARGET_IMAGE that refers to SOURCE_IMAGE use:
docker tag SOURCE_IMAGE[:TAG] TARGET_IMAGE[:TAG]
[https://docs.docker.com/engine/reference/commandline/tag/]{.ul}
Explanation
The default filename is Dockerfile (without an extension), and using the default can make various tasks easier while working with containers.
However, you can name it whatever you like.
For example, check the following output:
[https://stackoverflow.com/questions/26077543/how-to-name-dockerfiles]{.ul}
Explanation
The WORKDIR instruction can be used multiple times in a Dockerfile.
· If a relative path is provided, it will be relative to the path of the previous WORKDIR instruction.
· If the WORKDIR doesn't exist, it will be created even if it's not used in any subsequent Dockerfile instruction.
The WORKDIR instruction can resolve environment variables previously set using ENV. You can only use environment variables explicitly set in the Dockerfile. For example:
ENV DIRPATH /path
WORKDIR $DIRPATH/$DIRNAME
RUN pwd
[https://docs.docker.com/engine/reference/builder/#workdir]{.ul}
Explanation
Warning
When building an image, do not use your root directory, /, as the PATH as it causes the build to transfer the entire contents of your hard drive to the Docker daemon.
[https://docs.docker.com/engine/reference/builder/]{.ul}
Explanation
The HEALTHCHECK instruction tells Docker how to test a container to check that it is still working. This can detect cases such as a web server that is stuck in an infinite loop and unable to handle new connections, even though the server process is still running.
[https://docs.docker.com/engine/reference/builder/#healthcheck]{.ul}
Explanation
To create a new image from a container's changes use:
docker commit [OPTIONS] CONTAINER [REPOSITORY[:TAG]]
It can be useful to commit a container's file changes or settings into a new image.
This allows you to export a working dataset to another server.
Still, it is better to use Dockerfiles to manage your images in a documented and maintainable way. The Version Control can be used mainly for text-based files such as Dockerfiles, docker-compose.yml, and configuration files. Small binaries can also be kept in the same version control. Examples of version control are git, svn, Team Foundation Server, Azure DevOps, and Clear Case.
[https://success.docker.com/article/dev-pipeline]{.ul}
Explanation
Docker can build images automatically by reading the instructions from a Dockerfile.
A Dockerfile is a text document that contains all the commands a user could call on the command line to assemble an image.
Using docker build users can create an automated build that executes several command-line instructions in succession.
[https://docs.docker.com/engine/reference/builder/]{.ul}
Explanation
LABEL
LABEL <key>=<value> <key>=<value> <key>=<value> ...
The LABEL instruction adds metadata to an image. A LABEL is a key-value pair.
Example:
LABEL version="1.0"
[https://docs.docker.com/engine/reference/builder/#label]{.ul}
Explanation
A container is a runtime instance of a docker image.
A Docker container consists of
· A Docker image
· An execution environment
· A standard set of instructions
The concept is borrowed from Shipping Containers, which define a standard to ship goods globally. Docker defines a standard to ship software.
[https://docs.docker.com/glossary/]{.ul}
Explanation
EXPOSE <port> [<port>/<protocol>...]
The EXPOSE instruction informs Docker that the container listens on the specified network ports at runtime. You can specify whether the port listens on TCP or UDP, and the default is TCP if the protocol is not specified.
The EXPOSE instruction does not actually publish the port. It functions as a type of documentation between the person who builds the image and the person who runs the container, about which ports are intended to be published. To actually publish the port when running the container, use the -p flag on docker run to publish and map one or more ports, or the -P flag to publish all exposed ports and map them to high-order ports.
https://docs.docker.com/engine/reference/builder/#expose
The ENV instruction sets the environment variable <key> to the value <value>. This value will be in the environment for all subsequent instructions in the build stage.
Explanation
ENV
ENV <key> <value>
ENV <key>=<value> ...
The ENV instruction sets the environment variable <key> to the value <value>. This value will be in the environment for all subsequent instructions in the build stage.
Also, the environment variables set using ENV will persist when a container is run from the resulting image. You can view the values using docker inspect, and change them using docker run --env <key>=<value>.
https://docs.docker.com/engine/reference/builder/#env
Explanation
CMD instruction allows you to set a default command, which will be executed only when you run container without specifying a command. If Docker container runs with a command, the default command will be ignored. If Dockerfile has more than one CMD instruction, all but last CMD instructions are ignored.
ENTRYPOINT instruction allows you to configure a container that will run as an executable. It looks similar to CMD, because it also allows you to specify a command with parameters. The difference is ENTRYPOINT command and parameters are not ignored when Docker container runs with command line parameters. (There is a way to ignore ENTTRYPOINT, but it is unlikely that you will do it.)
https://docs.docker.com/engine/reference/builder
Explanation
Note: Don't confuse RUN with CMD. RUN actually runs a command and commits the result; CMD does not execute anything at build time, but specifies the intended command for the image.
[https://docs.docker.com/engine/reference/builder/#cmd]{.ul}
Explanation
To create a tag TARGET_IMAGE that refers to SOURCE_IMAGE use:
docker tag SOURCE_IMAGE[:TAG] TARGET_IMAGE[:TAG]
Example:
To tag a local image with name "myimage" into the "myrepositroy" repository with "version1.0":
docker tag myimage myrepositroy/myimage:version1.0
[https://docs.docker.com/engine/reference/commandline/tag/]{.ul}
Explanation
docker images [OPTIONS] [REPOSITORY[:TAG]]
The default docker images will show all top-level images, their repository and tags, and their size.
You can use --all, -a to show all images (default hides intermediate images)
[https://docs.docker.com/engine/reference/commandline/images/]{.ul}
Explanation
In the Dockerfile context, The RUN instruction will execute any commands in a new layer on top of the current image and commit the results.
Layering RUN instructions and generating commits conforms to the core concepts of Docker where commits are cheap and containers can be created from any point in an image's history, much like source control.
https://docs.docker.com/engine/reference/builder/#run
Explanation
To display detailed information on one or more images use:
docker image inspect [OPTIONS] IMAGE [IMAGE...]
[https://docs.docker.com/engine/reference/commandline/image_inspect/]{.ul}
Explanation
Docker inspect provides detailed information on constructs controlled by Docker.
By default, docker inspect will render results in a JSON array.
To display detailed information on one or more images use:
docker image inspect [OPTIONS] IMAGE [IMAGE...]
The options are:
--format , -f
Format the output using the given Go template
For the most part, you can pick out any field from the JSON in a fairly straightforward manner using the --format parameter.
$ docker inspect --format='{{.LogPath}}' $INSTANCE_ID
[https://docs.docker.com/engine/reference/commandline/inspect/]{.ul}
Explanation
The build is run by the Docker daemon, not by the CLI.
The first thing a build process does is send the entire context (recursively) to the daemon.
[https://docs.docker.com/engine/reference/builder/]{.ul}
Explanation
By default, when you push an image to DTR, the Docker CLI client doesn't sign the image.
You can configure the Docker CLI client to sign the images you push to DTR. This allows whoever pulls your image to validate if they are getting the image you created, or a forged one.
To sign an image you can run:
export DOCKER_CONTENT_TRUST=1
docker push <dtr-domain>/<repository>/<image>:<tag>
This pushes the image to DTR and creates trust metadata. It also creates public and private key pairs to sign the trust metadata, and push that metadata to the Notary Server internal to DTR.
Sign images that UCP can trust
With the command above you can sign your DTR images, but UCP doesn't trust them because it can't tie the private key you're using to sign the images to your UCP account.
To sign images in a way that UCP trusts them you need to:
· Configure your Notary client
· Initialize trust metadata for the repository
· Delegate signing to the keys in your UCP client bundle
[https://github.com/docker/docker.github.io]{.ul}
Explanation
Prune images
The docker image prune command allows you to clean up unused images. By default, docker image prune only cleans up dangling images. A dangling image is one that is not tagged and is not referenced by any container. To remove dangling images:
$ docker image prune
To remove all images which are not used by existing containers, use the -a flag:
$ docker image prune -a
[https://docs.docker.com/config/pruning/#prune-images]{.ul}
Explanation
EXPOSE <port> [<port>/<protocol>...]
The EXPOSE instruction informs Docker that the container listens on the specified network ports at runtime. You can specify whether the port listens on TCP or UDP, and the default is TCP if the protocol is not specified.
The EXPOSE instruction does not actually publish the port. It functions as a type of documentation between the person who builds the image and the person who runs the container, about which ports are intended to be published. To actually publish the port when running the container, use the -p flag on docker run to publish and map one or more ports, or the -P flag to publish all exposed ports and map them to high-order ports.
https://docs.docker.com/engine/reference/builder/#expose
Explanation
When an image is updated or rebuilt, only layers that change need to be updated, and unchanged layers are cached locally. This is part of why Docker images are so fast and lightweight. The sizes of each layer add up to equal the size of the final image.
An image does not have state and it never changes.
[https://docs.docker.com/glossary/]{.ul}
FROM can appear multiple times within a single Dockerfile to create multiple images or use one build stage as a dependency for another.
[https://docs.docker.com/engine/reference/builder/#from]{.ul}
Explanation
To push an image or a repository to a registry use:
docker image push [OPTIONS] NAME[:TAG]
[https://docs.docker.com/engine/reference/commandline/image_push/]{.ul}
Explanation
The FROM instruction initializes a new build stage and sets the Base Image for subsequent instructions. As such, a valid Dockerfile must start with a FROM instruction.
[https://docs.docker.com/engine/reference/builder/#from]{.ul}
Explanation
The WORKDIR instruction can be used multiple times in a Dockerfile.
If a relative path is provided, it will be relative to the path of the previous WORKDIR instruction. For example:
WORKDIR /a
WORKDIR b
WORKDIR c
RUN pwd
The output of the final pwd command in this Dockerfile would be /a/b/c.
[https://docs.docker.com/engine/reference/builder/#workdir]{.ul}
Explanation
Prune unused Docker objects
Docker takes a conservative approach to cleaning up unused objects, such as images, containers, volumes, and networks: these objects are generally not removed unless you explicitly ask Docker to do so.
This can cause Docker to use extra disk space. For each type of object, Docker provides a prune command. In addition, you can use docker system prune to clean up multiple types of objects at once.
Prune images
The docker image prune command allows you to clean up unused images. By default, docker image prune only cleans up dangling images. A dangling image is one that is not tagged and is not referenced by any container. To remove dangling images:
$ docker image prune
To remove all images which are not used by existing containers, use the -a flag:
$ docker image prune -a
[https://docs.docker.com/config/pruning/#prune-images]{.ul}
Explanation
You can examine all the layers in an image by using the docker image history command. It will display the following:
· Abbreviated layer ID
· Age of the layer
· Initial command of the creating container
· Total file size of that layer
[https://docs.docker.com/engine/reference/commandline/history/]{.ul}
Explanation
The WORKDIR instruction can be used multiple times in a Dockerfile.
If a relative path is provided, it will be relative to the path of the previous WORKDIR instruction. For example:
WORKDIR /a
WORKDIR b
WORKDIR c
RUN pwd
The output of the final pwd command in this Dockerfile would be /a/b/c.
[https://docs.docker.com/engine/reference/builder/#workdir]{.ul}
Explanation
The RUN instruction will execute any commands in a new layer on top of the current image and commit the results. The resulting committed image will be used for the next step in the Dockerfile.
Layering RUN instructions and generating commits conforms to the core concepts of Docker where commits are cheap and containers can be created from any point in an image's history, much like source control.
Therefore, it's ok to have more than one RUN instruction.
https://docs.docker.com/engine/reference/builder/#run
Explanation
ARG <name>[=<default value>]
The ARG instruction defines a variable that users can pass at build-time to the builder with the docker build command using the --build-arg <varname>=<value> flag.
https://docs.docker.com/engine/reference/builder/#arg
Unlike an ARG instruction, ENV values are always persisted in the built image.
The variable expansion technique in this example allows you to pass arguments from the command line and persist them in the final image by leveraging the ENV instruction.
https://docs.docker.com/engine/reference/builder/#run#using-arg-variables
Explanation
LABEL
LABEL <key>=<value> <key>=<value> <key>=<value> ...
The LABEL instruction adds metadata to an image. A LABEL is a key-value pair.
Example:
LABEL version="1.0"
[https://docs.docker.com/engine/reference/builder/#label]{.ul}
Explanation
The RUN instruction will execute any commands in a new layer on top of the current image and commit the results. The resulting committed image will be used for the next step in the Dockerfile.
ENTRYPOINT instruction allows you to configure a container that will run as an executable. It looks similar to CMD, because it also allows you to specify a command with parameters. The difference is ENTRYPOINT command and parameters are not ignored when Docker container runs with command line parameters. (There is a way to ignore ENTTRYPOINT, but it is unlikely that you will do it.)
CMD instruction allows you to set a default command, which will be executed only when you run container without specifying a command. If Docker container runs with a command, the default command will be ignored. If Dockerfile has more than one CMD instruction, all but last CMD instructions are ignored.
EXEC is not a Dockerfile instruction.
https://docs.docker.com/engine/reference/builder/
Explanation
To remove one or more images, use the following command:
docker image rm [OPTIONS] IMAGE [IMAGE...]
[https://docs.docker.com/engine/reference/commandline/image_rm/]{.ul}
Explanation
FROM can appear multiple times within a single Dockerfile to create multiple images or use one build stage as a dependency for another.
Each FROM instruction clears any state created by previous instructions.
[https://docs.docker.com/engine/reference/builder/#from]{.ul}
You use multiple FROM statements in your Dockerfile for multi-stage builds. Each FROM instruction can use a different base, and each of them begins a new stage of the build. You can selectively copy artifacts from one stage to another, leaving behind everything you don't want in the final image.
[https://docs.docker.com/develop/develop-images/multistage-build/#use-multi-stage-builds]{.ul}
Explanation
To create a tag TARGET_IMAGE that refers to SOURCE_IMAGE use:
docker tag SOURCE_IMAGE[:TAG] TARGET_IMAGE[:TAG]
Examples:
Tag an image referenced by ID
To tag a local image with ID "0e5574283393" into the "fedora" repository with "version1.0":
$ docker tag 0e5574283393 fedora/httpd:version1.0
Tag an image referenced by Name
To tag a local image with name "httpd" into the "fedora" repository with "version1.0":
$ docker tag httpd fedora/httpd:version1.0
Note that since the tag name is not specified, the alias is created for an existing local version httpd:latest.
Tag an image referenced by Name and Tag
To tag a local image with name "httpd" and tag "test" into the "fedora" repository with "version1.0.test":
$ docker tag httpd:test fedora/httpd:version1.0.test
[https://docs.docker.com/engine/reference/commandline/tag/#examples]{.ul}
Explanation
The RUN instruction will execute any commands in a new layer on top of the current image and commit the results. The resulting committed image will be used for the next step in the Dockerfile.
CMD instruction allows you to set a default command, which will be executed only when you run container without specifying a command. If Docker container runs with a command, the default command will be ignored. If Dockerfile has more than one CMD instruction, all but last CMD instructions are ignored.
https://docs.docker.com/engine/reference/builder/
Explanation
You can use an ARG or an ENV instruction to specify variables that are available to the RUN instruction.
Environment variables defined using the ENV instruction always override an ARG instruction of the same name.
https://docs.docker.com/engine/reference/builder/#run#using-arg-variables
Explanation
The ADD instruction operates similarly to the COPY instruction with two important differences.
The ADD instruction will additionally support:
· Fetch remote source files if a URL is specified
· Auto-extraction of archive files. Extract the files of any source that are determined to be of archive file type.
[https://docs.docker.com/engine/reference/builder/#add]{.ul}
Explanation
The ADD instruction operates similarly to the COPY instruction with two important differences.
The ADD instruction will additionally support:
· Fetch remote source files if a URL is specified
· Auto-extraction of archive files. Extract the files of any source that are determined to be of archive file type.
[https://docs.docker.com/engine/reference/builder/#add]{.ul}
Explanation
Docker Content Trust (DCT) allows you to verify the authenticity, integrity, and publication date of Docker images that are made available on the Docker Hub Registry.
By default, Content Trust is disabled. To enable Content Trust for signing and verifying Docker images that you build, push to, or pull from the Docker Hub, set the DOCKER_CONTENT_TRUST environment variable, for example:
export DOCKER_CONTENT_TRUST=1
[https://docs.oracle.com/cd/E37670_01/E75728/html/section-xnz_cxk_ht.html]{.ul}
Explanation
The FROM instruction initializes a new build stage and sets the Base Image for subsequent instructions. As such, a valid Dockerfile must start with a FROM instruction.
[https://docs.docker.com/engine/reference/builder/#from]{.ul}
Explanation
The FROM instruction initializes a new build stage and sets the Base Image for subsequent instructions.
As such, a valid Dockerfile must start with a FROM instruction.
ARG is the only instruction that may precede FROM in the Dockerfile.
For example:
ARG CODE_VERSION=latest
FROM base:${CODE_VERSION}
[https://docs.docker.com/engine/reference/builder/#from]{.ul}
Explanation
ENV
ENV <key> <value>
ENV <key>=<value> ...
The ENV instruction sets the environment variable <key> to the value <value>. This value will be in the environment for all subsequent instructions in the build stage.
Also, the environment variables set using ENV will persist when a container is run from the resulting image. You can view the values using docker inspect, and change them using docker run --env <key>=<value>.
https://docs.docker.com/engine/reference/builder/#env
Explanation
To search the Docker Hub for images use:
docker search [OPTIONS] TERM
The filtering flag (-f or --filter) format is a key=value pair. If there is more than one filter, then pass multiple flags (e.g. --filter is-automated=true --filter stars=3)
The currently supported filters are:
stars (int - number of stars the image has)
is-automated (boolean - true or false) - is the image automated or not
is-official (boolean - true or false) - is the image official or not
[https://docs.docker.com/engine/reference/commandline/search/#filtering]{.ul}
Explanation
By default, users with read and write access to a repository can push the same tag multiple times to that repository. For example, when user A pushes an image to library/wordpress:latest, there is no preventing user B from pushing an image with the same name but a completely different functionality. This can make it difficult to trace the image back to the build that generated it.
To prevent tags from being overwritten, you can configure a repository to be immutable in the DTR web UI. Once configured, DTR will not allow anyone else to push another image tag with the same name.
[https://github.com/docker/docker.github.io]{.ul}
Explanation
The main purpose of a CMD is to provide defaults for an executing container. These defaults can include an executable, or they can omit the executable, in which case you must specify an ENTRYPOINT instruction as well.
If CMD is used to provide default arguments for the ENTRYPOINT instruction, both the CMD and ENTRYPOINT instructions should be specified with the JSON array format.
[https://docs.docker.com/engine/reference/builder/#cmd]{.ul}
Explanation
The WORKDIR instruction sets the working directory for any RUN, CMD, ENTRYPOINT, COPY and ADD instructions that follow it in the Dockerfile. If the WORKDIR doesn't exist, it will be created even if it's not used in any subsequent Dockerfile instruction.
[https://docs.docker.com/engine/reference/builder/#workdir]{.ul}
Explanation
An ENTRYPOINT allows you to configure a container that will run as an executable.
You can override the ENTRYPOINT instruction using the docker run --entrypoint flag.
[https://docs.docker.com/engine/reference/builder/#entrypoint]{.ul}
The main purpose of a CMD is to provide defaults for an executing container. These defaults can include an executable, or they can omit the executable, in which case you must specify an ENTRYPOINT instruction as well.
If CMD is used to provide default arguments for the ENTRYPOINT instruction, both the CMD and ENTRYPOINT instructions should be specified with the JSON array format.
The CMD instruction has three forms:
· CMD ["executable","param1","param2"] (exec form, this is the preferred form)
· CMD ["param1","param2"] (as default parameters to ENTRYPOINT)
· CMD command param1 param2 (shell form)
[https://docs.docker.com/engine/reference/builder/#cmd]{.ul}
Explanation
Delete signed images
DTR only allows deleting images if the image has not been signed. You first need to delete all the trust data associated with the image before you are able to delete the image.
[https://github.com/docker/docker.github.io]{.ul}
Explanation
Docker builds images automatically by reading the instructions from a Dockerfile text file that contains all commands, in order, needed to build a given image.
A Docker image consists of read-only layers each of which represents a Dockerfile instruction.
[https://docs.docker.com/develop/develop-images/dockerfile_best-practices/]{.ul}
Explanation
Best practices for writing Dockerfiles:
· Create ephemeral containers
· Understand build context
· Pipe Dockerfile through stdin
· Exclude with .dockerignore
· Use multi-stage builds
· Don't install unnecessary packages
· Decouple applications
· Minimize the number of layers
· Sort multi-line arguments
· Leverage build cache
Explanation
Multi-stage builds allow you to drastically reduce the size of your final image, without struggling to reduce the number of intermediate layers and files.
With multi-stage builds, you use multiple FROM statements in your Dockerfile. Each FROM instruction can use a different base, and each of them begins a new stage of the build. You can selectively copy artifacts from one stage to another, leaving behind everything you don't want in the final image.
[https://docs.docker.com/develop/develop-images/multistage-build/#use-multi-stage-builds]{.ul}
Explanation
VOLUME ["/data"]
The VOLUME instruction creates a mount point with the specified name and marks it as holding externally mounted volumes from native host or other containers. The value can be a JSON array, VOLUME ["/var/log/"], or a plain string with multiple arguments, such as VOLUME /var/log or VOLUME /var/log /var/db.
[https://docs.docker.com/engine/reference/builder/#volume]{.ul}
Explanation
In an image, a layer is modification to the image, represented by an instruction in the Dockerfile. Layers are applied in sequence to the base image to create the final image.
When an image is updated or rebuilt, only layers that change need to be updated, and unchanged layers are cached locally. This is part of why Docker images are so fast and lightweight. The sizes of each layer add up to equal the size of the final image.
[https://docs.docker.com/glossary/]{.ul}
Explanation
Docker Hub is a service provided by Docker for finding and sharing container images with your team. It provides the following major features:
· Repositories: Push and pull container images.
· Teams & Organizations: Manage access to private repositories of container images.
· Official Images: Pull and use high-quality container images provided by Docker.
· Publisher Images: Pull and use high-quality container images provided by external vendors. Certified images also include support and guarantee compatibility with Docker Enterprise.
· Builds: Automatically build container images from GitHub and Bitbucket and push them to Docker Hub.
· Webhooks: Trigger actions after a successful push to a repository to integrate Docker Hub with other services.