Dev Container Features: Self Authoring Template

This repo provides a starting point and example for creating your own custom dev container Features, hosted for free on GitHub Container Registry. The example in this repository follows the dev container Feature distribution specification.

To provide feedback to the specification, please leave a comment on spec issue #70. For more broad feedback regarding dev container Features, please see spec issue #61.

Example Contents

This repository contains a collection of two Features - hello and color. These Features serve as simple feature implementations. Each sub-section below shows a sample devcontainer.json alongside example usage of the Feature.

hello

Running hello inside the built container will print the greeting provided to it via its greeting option.

{
    "image": "mcr.microsoft.com/devcontainers/base:ubuntu",
    "features": {
        "ghcr.io/devcontainers/feature-starter/hello:1": {
            "greeting": "Hello"
        }
    }
}

$ hello

Hello, user.

color

Running color inside the built container will print your favorite color to standard out.

{
    "image": "mcr.microsoft.com/devcontainers/base:ubuntu",
    "features": {
        "ghcr.io/devcontainers/feature-starter/color:1": {
            "favorite": "green"
        }
    }
}

$ color

my favorite color is green

Repo and Feature Structure

Similar to the devcontainers/features repo, this repository has a src folder. Each Feature has its own sub-folder, containing at least a devcontainer-feature.json and an entrypoint script install.sh.

├── src
│   ├── hello
│   │   ├── devcontainer-feature.json
│   │   └── install.sh
│   ├── color
│   │   ├── devcontainer-feature.json
│   │   └── install.sh
|   ├── ...
│   │   ├── devcontainer-feature.json
│   │   └── install.sh
...

An implementing tool will composite the documented dev container properties from the feature's devcontainer-feature.json file, and execute in the install.sh entrypoint script in the container during build time. Implementing tools are also free to process attributes under the customizations property as desired.

Options

All available options for a Feature should be declared in the devcontainer-feature.json. The syntax for the options property can be found in the devcontainer Feature json properties reference.

For example, the color feature provides an enum of three possible options (red, gold, green). If no option is provided in a user's devcontainer.json, the value is set to "red".

{
    // ...
    "options": {
        "favorite": {
            "type": "string",
            "enum": [
                "red",
                "gold",
                "green"
            ],
            "default": "red",
            "description": "Choose your favorite color."
        }
    }
}

Options are exported as Feature-scoped environment variables. The option name is captialized and sanitized according to option resolution.

#!/bin/bash

echo "Activating feature 'color'"
echo "The provided favorite color is: ${FAVORITE}"

...

Distributing Features

Versioning

Features are individually versioned by the version attribute in a Feature's devcontainer-feature.json. Features are versioned according to the semver specification. More details can be found in the dev container Feature specification.

Publishing

NOTE: The Distribution spec can be found here.

While any registry implementing the OCI Distribution spec can be used, this template will leverage GHCR (GitHub Container Registry) as the backing registry.

Features are meant to be easily sharable units of dev container configuration and installation code.

This repo contains a GitHub Action workflow that will publish each feature to GHCR. By default, each Feature will be prefixed with the <owner/<repo> namespace. For example, the two Features in this repository can be referenced in a devcontainer.json with:

ghcr.io/devcontainers/feature-starter/color:1
ghcr.io/devcontainers/feature-starter/hello:1

The provided GitHub Action will also publish a third "metadata" package with just the namespace, eg: ghcr.io/devcontainers/feature-starter. This contains information useful for tools aiding in Feature discovery.

'devcontainers/feature-starter' is known as the feature collection namespace.

Marking Feature Public

Note that by default, GHCR packages are marked as private. To stay within the free tier, Features need to be marked as public.

This can be done by navigating to the Feature's "package settings" page in GHCR, and setting the visibility to 'public`. The URL may look something like:

https://github.com/users/<owner>/packages/container/<repo>%2F<featureName>/settings

Adding Features to the Index

If you'd like your Features to appear in our public index so that other community members can find them, you can do the following:

• Go to github.com/devcontainers/devcontainers.github.io
  • This is the GitHub repo backing the containers.dev spec site
• Open a PR to modify the collection-index.yml file

This index is from where supporting tools like VS Code Dev Containers and GitHub Codespaces surface Features for their dev container creation UI.

Using private Features in Codespaces

For any Features hosted in GHCR that are kept private, the GITHUB_TOKEN access token in your environment will need to have package:read and contents:read for the associated repository.

Many implementing tools use a broadly scoped access token and will work automatically. GitHub Codespaces uses repo-scoped tokens, and therefore you'll need to add the permissions in devcontainer.json

An example devcontainer.json can be found below.

{
    "image": "mcr.microsoft.com/devcontainers/base:ubuntu",
    "features": {
     "ghcr.io/my-org/private-features/hello:1": {
            "greeting": "Hello"
        }
    },
    "customizations": {
        "codespaces": {
            "repositories": {
                "my-org/private-features": {
                    "permissions": {
                        "packages": "read",
                        "contents": "read"
                    }
                }
            }
        }
    }
}