This addon will install Docker 1.3.0 from https://get.docker.io/builds/Linux/x86_64 and will set things up for running nested Docker containers based on https://github.com/jpetazzo/dind.
In order to use it, you need to provide both the --privileged flag and because
/var/lib/docker cannot be on AUFS, so we need to make it a volume with -v /var/lib/docker.
To install it you can run configure-addons docker from within the container.
To specify a Docker version, use the the DEVSTEP_DOCKER_VERSION environmental
variable.