Add more subtleties about local testing and docker in Contributing docs page

[emiliom]

See https://echopype.readthedocs.io/en/stable/contributing.html#running-the-tests

• setup-services.py --deploy creates a fresh new image every time it's run
• To reuse a stopped an echopype tests:

  # see all the stopped containers
  docker ps -a
  # (re)start an existing docker container
  docker start <container id>

• setup-services.py --tear-down doesn't remove the docker image and volume that were created
• It's good to clean up your docker system time to time using docker system prune. They can take up a lot of space and potentially cause problems (we may have run into an issue with minio and the number of docker volumes).

cc @lsetiawan