Run impalad minicluster processes inside docker containers

I'm looking on some docker-related tasks, e.g. IMPALA-7941 and it'd be cool to be able to start up a minicluster with each impala process inside its own container. This is distinct from test-with-docker which runs the whole minicluster inside a container and is much closer to how containerised services are deployed in production.

I'm imagining this would be a start-impala-cluster.py flag.