Feature request: allow custom URL for manager client site

I had trouble with the default manager.com URL for running a local instance of the Manager client site. Instead of hardcoding a URL for this secondary site, what about allowing the user to choose their local URL, like with the additional sites feature? I don't know enough about Docker to be able to implement this myself.

Start by comparing how the manager client site's URL is configured with the existing additional sites feature, then trace the Docker configuration that launches the secondary site. Done means a user can choose a local URL for the manager client site instead of relying on the hardcoded manager.com URL.

Written by the indexing model from the issue text.