Build and push a Docker image with the gems for the CNG pipeline

What does this MR do?

This builds a Docker image with the gems bundle so that the CNG pipeline triggered by the review-build-cng job can pull it and save a few minutes (approx. 4 or 5 minutes).

The CNG support is done in gitlab-org/build/CNG!306 (closed).

Samples

Here you can see that gitlab-rails-ee finished in 12m 51s (instead of usually 17 minutes: https://app.periscopedata.com/app/gitlab/496118/Engineering-Productivity-Sandbox?widget=7014205&udv=785399) by pulling the image built by the build-gems-bundle-image job: https://gitlab.com/gitlab-org/build/CNG-mirror/-/jobs/338815709 🎉

Does this MR meet the acceptance criteria?

Conformity

• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides