fix(flax): use new flax image that manages its own startup

See #1559 (closed).

The root problem was my use of entrypoint in the docker-compose files, which overrode the Flax Dockerfile's CMD startup command. The solution is to make the Flax image responsible for the full startup sequence including wait-for-it.sh.

I also used supervisord for further assurance that the services will restart if they halt for some reason. I added this before I realised what the root cause was, but I feel it's a valuable addition and I've tested it, so I'm leaving it in.

This has a corresponding Flax MR flax!40 (merged)