fleet: add Dockerfile

This Dockerfile builds the fleet project from scratch, allowing
us to containerize the project with

    $ docker build -t coreos/fleet .

You can interact with fleetctl via

    $ docker run coreos/fleet fleetctl help

It probably doesn't make sense to containerize `fleet` itself since it relies on systemd but just throwing it out there.

Source code for `deis/go` can be found here: https://github.com/deis/dockerfiles/tree/master/go