Run tests against GitLab EE Docker image created by Omnibus nightly too
Docker images built nightly by Omnibus for GitLab CE are now available, and we run tests against most recent images, but those are still not available for GitLab EE (no gitlab-ee:nightly
or packages built by omnibus nightly).
This is currently blocked by https://gitlab.com/gitlab-org/omnibus-gitlab/issues/1440