Allow processes to stop on docker stop

It seems currently we are not gracefully terminating runit, and its child processes.

More discussion:

• https://meta.discourse.org/t/runsv-hanging-on-docker-container-shutdown/36844
• https://engineeringblog.yelp.com/2016/01/dumb-init-an-init-for-docker.html

There are some scripts available to address this:

• https://github.com/QuantumObject/docker-baseimage/blob/master/bin/my_init
• https://github.com/discourse/discourse_docker/blob/master/image/base/boot (https://github.com/discourse/discourse_docker/commit/d821539c6a63d1fbeaf9f56811aaf9b2be11185d)
• https://github.com/krallin/tini