Clarify shell and Docker executor runner setup

What does this Merge Request do and why?

!2386 (merged) dropped the setup steps for the Docker configuration, but it wasn't clear the registration process was the same for both shell and Docker executors.

We got rid of the use of registering with the Docker container because it's easier to download the binary and have it write the initial config file directly into the local path.

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.