"Disable Container Registry but use GitLab as an auth endpoint" - question

I'm trying to make gitlab work with an external registry so that users can use their gitlab credentials with "docker login".

I read the documentation (https://docs.gitlab.com/ce/administration/container_registry.html) and something is not clear to me... Under "Disable Container Registry but use GitLab as an auth endpoint", you provide a way for gitlab and the external registry to communicate, which is fine. But I don't see how gitlab users can authenticate with "docker login " unless the external registry is using gitlab as the authentication provider.

If the external registry should use gitlab as the authentication provider, is it documented anywhere? If not, then what am I missing?