Gitlab CI image build + push

This Patch will enable building and pushing Docker images to the Gitlab registry. The CI script is configured to only build when the commit is tagged.

The tag string is used to find the relevant environment files in .devcontainer folder. Should they not exist than the build will error.

A limitation of this patch is that the username / namespace of the project (Dantali0n in my case) can not contain any uppercase characters. This is of course not applicable to the name tango-controls hence why I did not bother implementing a string to lower conversion.

Since testing .gitlab-ci scripts requires spamming commits, for lack of a better term, I would recommend squashing this merge.