Make it possible to build new revisions of older versions of builder images

Omnibus-GitLab Builder images has its CI configured in such a way that

1. IMAGE_TAG defines what is the docker tag
2. Images are built and pushed on master branch

However, this makes it difficult if we want to modify the builder image for a previous GitLab version. It will include modifying master branch to how it was in the version that was used in that previous GitLab version, make changes and get it pushed, then reset master back to normal and repeat for next GitLab version etc.

Proposal

Move builder image build/push to tag pipelines from master. This branch off from an earlier commit and create a new tag to generate a new set of images without disturbing the master branch at all. The only downside is, after merging an MR, someone will have to create and push a tag. We can even add it as the last stage of master pipeline.