Build release-tools base image with gem dependencies

What does this MR do and why?

Our current base image is created with https://dev.gitlab.org/gitlab/gitlab-build-images/-/blob/master/Dockerfile.release-tools

• It makes harder to keep it in sync with .ruby-version
• It is not bundling gem dependencies, we will not be able to run our pipelines if a gem we utilize suddenly disappear from the internet
• It relies on our DEV instance docker registry instead of OPS

With this MR:

• we are moving the base image into release-tools, storing it on both GitLab.com and OPS.
• We bundle all our gems and rebuild the base image when a dependency changes
• we ensure we always run with the gem specified in the lockfile
• we test the new base image before making it the default version

Next steps after we merge this:

• merging !1805 (merged)
• add go to the base image for delivery-metrics

Author Check-list

• Has documentation been updated?