Docker executor in container launch

Note: I haven't embed the docker image or include Dockerfile and docker build as part of the build step yet, but this is the working code to put up review first.
What happens when this container exits? Don't you want to start reaping the pid and store the result in 'statuses'? We should probably add an explicit test that makes sure that when an executor container dies on it's own we notice as much and clean up.