Update docs to include gotcha note

What does this MR do and why?

This MR adds a note about using arbitrary port numbers for the container registry if that registry exists behind an AWS application load balancer which only listens on ports 80/443. This is especially useful if users rely on the docker login examples with the copy/paste icon on the registry page.

Screenshots or screen recordings

How to set up and validate locally

Add the following to /etc/gitlab/gitlab.rb

registry_external_url 'https://registry-gitlab.example.com'
registry_nginx['redirect_http_to_https'] = true
registry_nginx['listen_port'] = 4567
registry_nginx['listen_https'] = true

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.