Add a CI build test

My script contribution for 13.7, introduces a new script that is able to find the latest CNG release version on the CNG repo (for use in CI branches)

And adds a gitlab-ci for this repo. The ci has a fetch job that grabs the git repos once, and stores them in artifacts, then two tests, one that runs the release helper and diffs the output. And another that runs the release-helper then builds the docker containers.

Does not yet push the docker containers anywhere, do any testing of the containers, or handle any release tasks.

Next steps for the ci is likely to parallelised the build jobs, and push the images to this repos registry for further testing.