Gitlab registry does not support cross-repo mounting scopes.

When pushing to a registry with multiple scopes where some scopes are from external registries (cross-repo mounting) gitlab does not auth the blob or layer. I believe cross-repo mounting is part of the docker spec, so this should be supported.

https://github.com/gitlabhq/gitlabhq/blob/889098471e6efc5afa63d70d4089c52326fff43e/app/services/auth/container_registry_authentication_service.rb#L78

My company, Nordstrom, is a enterprise-edition customer running version 10.8.2-ee.

Thank you.