Add busybox rootfs so we can run containers

make sh will spawn a new container using the minimal.json file inside
busybox and you don't even know what is going on.
Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@docker.com> (github: crosbymichael)
Merge pull request #120 from crosbymichael/add-tests1

Add busybox rootfs so we can run containers