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)
(I'll dig for you, anyhow - this is a nice start :D)