Introduce gitlab-base image in order to release 15.1
In CNG!1034 (merged) a new base image was introduce.
For 15.1 this is used as the base image for the gitlab-ruby image. But the eventual intent is that some of the current images that use gitlab-ruby as a base, but don't actually need ruby, will be able to use it instead. (like shell and workhorse).
cc\ @SteveTerhar