Docker image builder for Gentoo stages

This is largely a helper tool for producing base images for Docker containers using musl-libc Gentoo stages. mkimg.sh takes a number of arguments on the command line:

$ sh mkimg.sh STAGE3 SNAPSHOT DOCKER_REPO DOCKER_BASENAME DOCKER_TAG

where:

• STAGE3: is the path to a stage 3 tarball
• SNAPSHOT: is a portage tree snapshot
• DOCKER_REPO: is the name of your docker repository or local registry
• DOCKER_BASENAME: is the name that will be prefixed to the names of all containers built.
• DOCKER_TAG: is the tag applied to the generated docker images.

In the current working directory, it will create the directories:

• packages/${TAG}: The binary package directory
• portage/${TAG}: The portage tree snapshot directory

mkimg.sh then imports the stage 3 tarball into a "raw" container which is used as an ephemeral build environment, the image will be tagged ${DOCKER_REPO}/${DOCKER_BASENAME}-raw:${DOCKER_TAG}.

Having imported that, a docker container is launched, in privileged mode (some packages will not build without) and mkimg-container.sh runs.

mkimg-container.sh rebuilds the entire system, doing an emerge -e @system @world to populate the binary packages. This step can also be adjusted to set USE flags as desired.

It then populates two staging directories:

• ${BASENAME}-dev-${TAG} is populated with all packages
• ${BASENAME}-rt-${TAG} is populated with only run-time packages

These are then packaged up as .tar files.

mkimg.sh takes over at this point, imports the two tars as docker images and publishes them.

Container usage

There are two containers produced by this that are intended for actual use. The dev container is used for building binaries. It has the full Gentoo suite available.

The rt container only contains the run-time environment. There's no compilers, and only minimal support packages. Portage works for merging binary packages produced by the development (dev) container.

The installation procedure is to use docker run with the dev container to compile your binary packages. In the absence of docker build -v support (see Docker bug #14080), I suggest this approach:

$ docker run --rm -v /usr/portage:/usr/portage \
	-v packages:/usr/portage/packages \
	myrepo/baseimage-dev:tag emerge www-servers/apache
$ docker run --name my-temp-apache-container --rm -v /usr/portage:/usr/portage \
	-v packages:/usr/portage/packages \
	myrepo/baseimage-rt:tag emerge -K www-servers/apache
$ docker commit my-temp-apache-container my-temp-apache-image

… then use that image with a Dockerfile to do final customisations.

In doing this, we do not pollute the target container with build-time dependencies and thus keep it as small as possible.