CNG: Remove dependencies to gitlab-go as the base image

Background

gitlab-go is based on gitlab-ruby and contains Go language compiler and tools which makes it suitable for build stages. It is already used in the build stage of a few images, including gitaly and gitlab-workhorse.

There are still images that use gitlab-go as the base image, namely gitlab-shell and gitlab-metrics-exporter. gitlab-org/build/CNG!1062 (merged), among other changes, removed the dependency of gitlab-shell but it has been reverted (see gitlab-org/build/CNG!1076 (merged) for details).

Opportunity

Removing gitlab-go as the base image and using gitlab-ruby instead, reduces the final image size with minimal impact on the final image. It also allows to move Go compiler and tools to a more relevant location.

Proposal

Use gitlab-ruby as the base image for:

• gitlab-metrics-exporter gitlab-org/build/CNG!1228 (merged)
• gitlab-shell gitlab-shell final image is now based on gitlab-base as of gitlab-org/build/CNG!1062 (merged)