Restrict omnibus self-managed registry test to pipeline main

What does this MR do?

This test is meant only to run in omnibus and not on subdomains. Using the tag only to restrict to the main pipeline. Container Registry tests running in dot_com environments need other configurations for docker-in-docker.

Failure: https://ops.gitlab.net/gitlab-org/quality/staging/-/jobs/4379554/artifacts/file/gitlab-qa-run-2021-07-19-05-33-31-6efa3057/gitlab-ee-qa-4f020aa2/qa-test-2021-07-19-05-33-41-2b1f4009378b2006/package_self-managed_container_registry_pushes_image_and_deletes_tag.png