added info on how to build and run the image
makefile to build the image and/or the buildroot fs
buildroot fs inside a docker container

Uses a docker image to build the root filesystem to be used later to
make the busybox image.

Credit to jpetazzo: https://github.com/jpetazzo/docker-busybox
Build the busybox with the cached rootfs

This is intended to be used from the Docker Registry Automated Build
(https://registry.hub.docker.com/u/jllopis/busybox)
cached busybox filesystem