docs: start a developers guide with a tip on sambacc builds

Start a developers guide document by documenting a process I use to test
out development branches of sambacc in a proper samba-container image.

Signed-off-by: John Mulligan <[email protected]>

docs/development.md

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+# Development Guide
+
+
+## Building samba containers with unreleased sambacc code
+
+Changes to `sambacc` are validated by a suite of unit tests to ensure a minium
+level of quality, but that is often not enough to fully validate a
+work-in-progress feature, especially one that needs to interact with components
+from Samba in complex ways. One may want to try out an unreleased branch of
+sambacc code as part of a samba container image. Two methods of doing this are:
+* Build sambacc RPMs and put them in a yum/dnf repo
+* Customize the Containerfile to use a sambacc build stage
+
+Both methods make use of the sambacc build image. The files needed to build the
+image are part of the [sambacc
+repo](https://github.com/samba-in-kubernetes/sambacc) and already-created
+images are available at quay.io:
+[quay.io/samba.org/sambacc](https://quay.io/repository/samba.org/sambacc).
+
+### RPMs
+
+One can build rpms using the sambacc test-and-build container.
+In this example we assume you have a git checkout of sambacc as the
+local path. Create a new directory to store build artifacts in:
+```
+mkdir -p _build
+```
+
+Then run the container command like follows:
+```
+podman run -v $PWD:/var/tmp/build/sambacc  -v $PWD/_build:/srv/dist/:z -e SAMBACC_DISTNAME=dev  quay.io/samba.org/sambacc:latest
+```
+
+Breaking it down, we're mounting the current dir at `/var/tmp/build/sambacc`,
+mounting the build dir at `/srv/dist` and telling the build container
+to store artifacts under the "distribution name" of `dev`. This should
+result in rpms, whl files and other artifacts in `_build/dev`. You can
+name your "dist" anything.
+
+Now you have a directory with rpms in it you can run `createrepo` on them
+and/or publish them on the web. Managing the rpms is an exercise left to the
+reader.
+
+To get them into a samba-container image, like the samba-server or
+samba-ad-server image, we need to get or create a repo file pointing to the
+repo hosting your rpms. The repo file must be saved into the build container at
+a path named like `/tmp/sambacc-dist-latest/sambacc*.repo`, so that the
+`install-sambacc.sh` script that is run during the image build can find it.
+
+Typically this means modifying the Containerfile. Here's an example modification
+to the `images/server/Containerfile.fedora` file:
+```
+COPY .common/install-sambacc-common.sh /usr/local/bin/install-sambacc-common.sh
+COPY install-sambacc.sh /usr/local/bin/install-sambacc.sh
+# Add an ADD command to copy our repofile into the build
+ADD https://my-cool-repo.example.org/mystuff/sambacc.repo /tmp/sambacc-dist-latest
+RUN /usr/local/bin/install-sambacc.sh \
+    "/tmp/sambacc-dist-latest" \
+    "${SAMBACC_VERSION_SUFFIX}"
+```
+
+Now build the image the usual way. It should contain your specific sambacc rpms.
+
+
+### Build Stage
+
+Rather than building the sambacc RPMs and creating a repo for them, the build
+steps can be combined by modifying the `Containerfile`s to add a specific build
+stage. First add the build stage to the top of the Containerfile:
+```
+# --- new stuff ---
+FROM quay.io/samba.org/sambacc:latest AS sccbuilder
+ARG SAMBACC_VER=my-cool-branch
+ARG SAMBACC_REPO=https://github.com/example-user/sambacc
+RUN SAMBACC_DISTNAME=latest \
+    /usr/local/bin/build.sh ${SAMBACC_VER} ${SAMBACC_REPO}
+# --- end new stuff ---
+
+FROM registry.fedoraproject.org/fedora:38
+```
+
+The variables `SAMBACC_VER` and `SAMBACC_REPO` can be overridden on the command
+line so you don't have to keep modifying the Containerfile to set them, unless
+you want to. `SAMBACC_VER` takes a git ref and that can be a barnch name or a
+commit hash. Using a commit hash can be handy to avoid caching issues.
+
+Next, we need to make a modification to the RUN command that executes
+`install-sambacc.sh`:
+```
+# add the --mount argument to map the dist dir of the sccbuilder
+# container to the /tmp/sambacc-dist-latest dir in the current build
+# container.
+RUN --mount=type=bind,from=sccbuilder,source=/srv/dist/latest,destination=/tmp/sambacc-dist-latest bash -x /usr/local/bin/install-sambacc.sh \
+    "/tmp/sambacc-dist-latest" \
+    "${SAMBACC_VERSION_SUFFIX}"
+```
+
+Very old versions of podman and docker may not support `--mount`. As an
+alternative, you can add a `COPY` command to copy the rpms from one container
+to the other.