Consider parameterizing and/or using a different port (other than 5000) for dev install

Working through the instructions for [Installing h in a development environment](https://h.readthedocs.io/en/latest/developing/install/) in mac os leads to multiple errors at the `make dev` step which include that the root of these errors is `[gunicorn.error:ERROR] Connection in use: ('0.0.0.0', 5000)`.

On macos, the system by default starts a listener at port 5000 for airplay. While not a "registered port", it is also used by many other systems e.g., for UPnP.

This of course doesn't affect running things under docker (where the ports are mapped differently), but makes it so that the above instructions cannot be followed directly for a substantial number of potential dev systems.

Ideally this would be controlled by an env parameter so developers could choose their own available port.

Another possibility would be to prepare instructions for a dev environment that runs everything (even the main app code, not just the services) within docker containers.