This introduces a Docker builder. The docker builder is able to create
containers by starting an existing Docker image, provisioning it using
standard practices, and then exporting it using `docker export`.

website: Fix missing whitespace. [ci skip]

/cc @mwhooker - CCing you on this because it is also ridiculous. See the
big comments

/cc @mwhooker - WOW. By luck, I had a hunch that maybe something like
this might be going on based on straces I was reading. Check:
https://github.com/dotcloud/docker/issues/2628

Anyways, this works now. No more blocker!