Backport: Add `release-environments` child pipeline and build images
Part of gitlab-com/gl-infra/delivery#2674 (closed)
In order to have commits to stable branches able to be deployed to release environments for deployment testing and validation, we need to build imags with the CNG project containing the contents of the stable branches.
This change adds the work to get images built through the CNG-mirror
project for use by the release environments. These will only be built
on commits to a stable branch.
This is a backport of the MR !110712 (merged). As the changes to CI only take place on stable branches, I am asking this to be pulled into 15-9-stable-ee
in order to test this ASAP, and ensure it won't cause issues with the 15.10 release stable branches. This will also unblock further release environments work