I ran across a similar thing a few months ago named docker-gen[1]. It listens to the docker event stream in the same way Registrator does, but instead of talking to an external service it regenerates a config file from a template and then optionally runs a command. The canonical example is regenerating a host-wide nginx proxy config when containers start and die, and then restarting nginx.

Just goes to show how versatile docker can be by exposing that event stream to arbitrary processes.

[1]: https://github.com/jwilder/docker-gen