support stop_grace_period and stop_signal

[jbrekelmans]

Kubernetes does not have an API to configure the signal sent to containers on shutdown, but docker-compose and Dockerfiles do. STOPSIGNAL in a Dockerfile is respected by Kubernetes, but stop_signal in docker-compose is not
stop_grace_period should be mapped to terminationGracePeriod.
We should give a warning if stop_signal in specified and not set to SIGTERM.

How are stop signals parsed?
https://github.com/moby/moby/blob/e105a74c54193e0ebdeddd93afc8746c33aeca73/container/container.go#L498
stop timeout vs stop_grace_period?