Update docs to include gotcha note

doc/administration/packages/container_registry.md

@@ -151,6 +151,19 @@ otherwise conflicts occur.
 
 If your certificate provider provides the CA Bundle certificates, append them to the TLS certificate file.
 
+An administrator may want the container registry listening on an arbitrary port such as `5678`.
+However, the registry and application server are behind an AWS application load balancer that only
+listens on ports `80` and `443`. The admin may simply remove the port number for
+`registry_external_url`, so HTTP or HTTPS is assumed. Then, the rules apply that map the load
+balancer to the registry from ports `80` or `443` to the arbitrary port. This is important if users
+rely on the `docker login` example in the container registry. Here's an example:
+
+```ruby
+registry_external_url 'https://registry-gitlab.example.com'
+registry_nginx['redirect_http_to_https'] = true
+registry_nginx['listen_port'] = 5678
+```
+
 **Installations from source**
 
 1. Open `/home/git/gitlab/config/gitlab.yml`, find the `registry` entry and