Use a *VERSION file for builder image revision
Right now we have BUILDER_IMAGE_REVISION: "0.0.5"
in our .gitlab-ci.yml
file.
This is hard to reference in documentation as it will change. We should just create a version file which we can read from the cloned repository. This way we can easily reference it without needing to know the version. Checking out the branch/tag will also make it simple to find the revision.