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)
Dude, this is already in busybox - no need to add it explicitly. :)