Start from macOS 10.15; Port 50051 is occupied by system

[maozhenyu123]

We might want to change pythonrcpserver port from 50051 to ?.

[robkooper]

I think there is no reason for port 50051 to be open to the world, is there? if it is inside the docker stack we should be able to have the containers talk to each other on the docker network.

[angrave]

This is just for development purposes when connecting to it from the outside. It would be useful to pick a default port that is not in use.