Simplify NGINX loopback interface instructions

What does this merge request do and why?

The previous NGINX instructions may lead to confusion since instructions for 127.1.1.1 don't discuss how to ensure that address stays around after a reboot. We already have instructions in local_network.md to use 127.16.123.1, so let's standardize on that address.

Also add a note that the step can be skipped unless you need to use a runner under Docker.

Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

Merge request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise an issue to follow-up.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.