Building using the Docker images

We have Docker images for a bunch of different Linux distros, so this patch adds a helper script for running buzzy inside of one of those images to build packages for a different distro than your native one.  As an example:

    $ buzzydock centos:centos7 install

The packages will end up in `$HOME/.cache/buzzy/distro/centos-centos7`.

This functionality depends on the `buzzy/[distro]` Docker images, which are automatically built by the Docker registry in response to change in the [buzzy-docker](/redjack/buzzy-docker) repository.