nspawn-runner: support for image selection

.gitlab-ci.yml supports 'image' to allow selecting in which environment the script gets run. The documentation says "Used to specify a Docker image to use for the job", but it's clearly a bug in the documentation, because we can do it with nspawn-runner, too.

It turns out that most of the environment variables available to CI runs are also available to custom runner scripts. In this case, the value passed as image can be found as $CUSTOM_ENV_CI_JOB_IMAGE in the custom runner scripts environment.

After some experimentation I made this commit that makes every chroot under /var/lib/nspawn-runner available as an image:

# Set up 3 new images for CI jobs:
nspawn-runner chroot-create buster
nspawn-runner chroot-create bullseye
nspawn-runner chroot-create sid

That's it, CI scripts can now use image: buster, image: bullseye or image: sid, as they please. You can manually set up other chroots under /var/lib/nspawn-runner and they'll be automatically available.

You can also now choose a default image in config.toml in case the CI script doesn't specify one:

prepare_args = ["--verbose", "prepare", "--default-image=buster"]