Draft: Manifest 4: Manifest Aliases

This is MR 4/4 for my proposed rework of gitlab-org/gitlab-runner!4680 (closed).

With the prior work, we have full control over the docker tags used throughout the process, so we can go back to the original x86_64 architecture label within the docker tags, and still create the appropriate manifest list. We can now also include the refTag and latest aliases for the manifest list, to keep the conventions consistent between helper builds and the manifest lists referring to them.