dockerfile builder aci

[philips]

As we have the build-repository docker ACI and this library we should build an ACI that can take a Dockerfile in a volume and then outputs the ACI into another volume.

The basic invocation for rkt would be something like:

$ git clone https://github.com/coreos/etcd
$ rkt run -volume dockerfile,kind=host,source=`pwd`/etcd -volume output,kind=host,source=path-to-output dockerfile2aci.aci