docker login returns error 500

While setting up the registry according to the manual https://github.com/sameersbn/docker-gitlab/blob/master/docs/container_registry.md I encountered a setup problem:
When trying to authorize a user via docker login, a 500 error occurs.
docker login registry.host.com -u user --password password returns Error response from daemon: Get "https://registry.host.com/v2/": received unexpected HTTP status: 500 Internal Server Error
At the same time, there are no entries in the docker logs about the cause of the error.
After mounting /var/logs/ on the host, I was able to find out the reason: there are no access rights to the file /certs/registry.key
After chown docker:docker registry.key in certs directory, everything worked as expected.
I think it is necessary to add a check for access rights to the file to the documentation, since when running gitlab according to this manual, the error is not displayed in the docker logs.

Start with docs/container_registry.md and review the documented Docker registry setup. Add a troubleshooting note covering permissions on /certs/registry.key, the resulting Docker login 500 error, and the required ownership change; done means users can diagnose this failure from the documentation.