Update container_registry.md

What does this MR do?

Add a warning about what ports to choose for the external registry URL

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

It would have saved me about 3 hours if this line was in the document.

What are the relevant issue numbers?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

It's just a one-line documentation addition.

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together

doc/administration/container_registry.md

@@ -122,6 +122,10 @@ Registry is exposed to the outside world is `4567`, here is what you need to set
 in `gitlab.rb` or `gitlab.yml` if you are using Omnibus GitLab or installed
 GitLab from source respectively.
+>**Note:**
+Be careful to choose a port different than the one that Registry listens to (`5000` by default),
+otherwise you will run into conflicts .
+
 ---
 **Omnibus GitLab installations**