Enable GCS support for docker registry

I'm using Google Cloud Storage as the backend for my docker image registry and currently have to rebuild the registry after each Gitlab upgrade. Could you please add the flag DOCKER_BUILDTAGS=include_gcs to the docker registry make execution?

The steps i'm currently using for upgrading is:

GOPATH=/go go get github.com/docker/distribution
cd /go/src/github.com/docker/distribution; git checkout v2.4.1
GOPATH=/go DOCKER_BUILDTAGS=include_gcs make clean binaries
gitlab-ctl stop registry
cp bin/* /opt/gitlab/embedded/bin/
gitlab-ctl start registry

I'm not sure where this flag would go in the omnibus repository. I was looking for it in order to make a PR, but it wasn't obvious where it would go.

Thanks