Add Dockerfile.

Complete with instructions on how to use. (see file)

Side note: Currently the go packages are being imported
relatively `./command`, which is certainly a thing in
python & javascript, but is a bit of a faux pas in go.

I could make a PR for that too, but I was more focused
on the Dockerfile & didn't want to cramp your style if
that's your thing. :)

Signed-off-by: Jessica Frazelle <jess@docker.com>