Merge branch 'hotfix/better-container-registry-documentation' into 'master'

Add some docs for Docker Registry configuration - Relatively important for anyone installing from source

There have been a few issues created because of the lack of documentation for how to configure Docker Registry when installing from source, specifically what the `realm` needs to be and the `service`

These 2 bits of info will stop a lot of headaches 👍 

Issues:
https://gitlab.com/gitlab-org/gitlab-ce/issues/17957
https://gitlab.com/gitlab-org/gitlab-ce/issues/17991

See merge request !4738

doc/administration/container_registry.md

@@ -85,6 +85,17 @@ GitLab does not ship with a Registry init file. Hence, [restarting GitLab][resta
 will not restart the Registry should you modify its settings. Read the upstream
 documentation on how to achieve that.
 
+The Docker Registry configuration will need `container_registry` as the service and `https://gitlab.example.com/jwt/auth` as the realm:
+
+```
+auth:
+  token:
+    realm: https://gitlab.example.com/jwt/auth
+    service: container_registry
+    issuer: gitlab-issuer
+    rootcertbundle: /root/certs/certbundle
+```
+
 ## Container Registry domain configuration
 
 There are two ways you can configure the Registry's external domain.