Document how to configure the package to use external registry

Bundled Container registry was built with one use case in mind: simplify the configuration neeeded when running both GitLab and Registry on a single instance.

It is however possible to configure GitLab using the package to connect to external registry:

https://docs.gitlab.com/ce/administration/container_registry.html#disable-container-registry-but-use-gitlab-as-an-auth-endpoint

We need to improve this documentation to make sure it is clear what needs to be set because currently the docs are not correct.

mentioned in issue gitlab-com/support#712

This still needs more work. Using the settings as described, the "enable registry" setting won't appear in project settings page.

@LongLiveCHIEF Welcoming MR's to improve the docs :)

@marin I've been poring over code trying to figure out how to fix, but I'm beginning to think it's possibly a code bug. the only way I can see to get enable registry to show up in project settings page, is to set registry['enable'] = true in /etc/gitlab/gitlab.rb.

Every test I've run, the gitlab_rails['enable_registry'] setting doesn't actually do anything.

@LongLiveCHIEF Oh, the setting is incorrect. It should actually be gitlab_rails['registry_enabled'] = true. I can see that is how it is noted down at https://gitlab.com/gitlab-org/gitlab-ce/blob/9ac39e008a0fbd5d54bade0fb75741bd73507cb7/doc/administration/container_registry.md#disable-container-registry-but-use-gitlab-as-an-auth-endpoint

I'm trying to reconcile the docs with what is supposed to happen with my configs. The rails keys just don't make sense, and I think the docs won't make sense until there is a code refactor. I have a few scenarios that I can describe, but haven't yet been able to successfully configure to achieve any of them.

as you were. PEBKAC.