Documentation for Registry and Mattermost behind External Proxy

For a bit of context, I'm currently running HAProxy that handles the SSL certificate and use it to proxy to the gitlab/gitlab-ce docker image.

Whereas the documentation describes how to proxy and handle SSL outside of GitLab, it does not cover the new Registry service nor Mattermost. Maybe I'm missing it, but would it be possible to get this added to the documentation for others who also run into the challenge of running behind a proxy?