Update Dockerfile and hack to support compiling device-mapper code statically (using go1.2rc1)

Lots of thanks to @creack for figuring out the hard parts via https://github.com/creack/docker_devmapper/blob/master/Dockerfile.

/cc @vieux @shykes @creack

Also, I've updated the packaging documentation in `hack` to reflect the newer requirements (hopefully didn't forget anything - so that needs more eyeballs on it).