Improve install guide for runner

What does this Merge Request do and why?

!2386 (merged) dropped the setup steps for the Docker configuration, but this made it unclear how to register a runner with Docker alone.

This commit shows how to register a runner using a Docker container while still guiding the user to use a GDK-managed runner. Registering with SSL enabled is also covered.

We also make it clearer that the executor should be specified.

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 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.