Expose docker on port 4243 so you can control it from os x docker client

The `boot2docker` command line use NAT to port `4243` but the docker daemon that runs in the image doesn't expose itself on 4243.

This PR exposes port 4243 for docker command such as from the new `0.7.3` docker client for Mac.

FWIW this is also what `docker-osx` does as well, but `boot2docker` is way more awesome in it's speed and minimalism! :-)