Build cloud native images on tags

What does this MR do?

When on a tag, trigger a multi-project pipeline in the CNG repostiory.

Opting for a trigger rather than an addition to our release-tools project for a few reasons:

• The Dockerfiles in the CNG image repo change infrequently, and as a result I don't feel the need/overhead for stable branches in that repo at this time
• My intent with the CNG repo, is that once stable, the Dockerfiles would actualy move to their component projects, to be versioned with the code they are building
• It is likely that we will want to followup with a manually triggered package for branches for devs, and possibly review apps, so it made sense to build the CNG ci jobs to accept this sort of pipeline.

Why was this MR needed?

For the Cloud Native charts Beta, we need to use versioned images so users aren't getting all the changes from master every time a container restarts.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• Conform by the merge request performance guides
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Part of https://gitlab.com/charts/gitlab/issues/232