Support a Docker HEALTHCHECK

[Rycochet]

As shown in #1628 (comment) curl is not included in the Docker image, which is one of the normal ways to check a web service is responding, so it's being manually installed.

So adding a request to either install the curl command in the Dockerfile so it's available and add it to the docs, or add a specific HEALTHCHECK support section to it so the container health is known.

Note that Docker itself does nothing with that beyond reporting on the status in docker ps etc, but using something like autoheal will allow unresponsive containers to be restarted.