I know about that, and we used to use that but had to move away from it because it created a lot of scalability problems (mind you, most of them due to disk usage).

With harbor we can save docker images layers to switft, the openstack flavor of object storage (S3). That solves a lot of scalability problems.

AFAIK gitlab ships the docker Registry underneath so the problems stay, mostly. I think that harbor does the same. I skimmed the harbor source and it seems that it forwards http requests to the docker registry if you hit the registry API endpoints.

Haven't looked at Quay but as far as I know wherever there's the docker registry you'll have garbage collection problems.

One note on the side: I think that quay missed their chance to become the goto docker registry. Red Hat basically open sourced it after harbor had been incubated in the cncf (unsurprisingly, harbor development has skyrocketed after that event).