ci: Use GitLab container registry

Instead of using pre-built containers hosted on Quay, build
containers as part of the GitLab CI pipeline and upload them to the
GitLab container registry for later use.

This will not significantly slow down builds, because containers are
only rebuilt when the corresponding Dockerfile has been modified.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>