Add Dockerfile.

I'm just going to leave this here.

You see I was trying to containerize a node webkit app, so I
thought why not atom. Except halfway through I realized it
actually isn't a node webkit app. I finished it anyways
because I had already started. It mounts the X11 socket
so it's a containerized desktop app.
You can do what you want with it.

But since this is OSS, I am going to give you my opinion
anyway. So here it is:
You should take this and use it as a build script, because
it seems like building this is a real PITA.

Ok that's all, enjoy.
Also this file was written in vim.
No, I'm not sorry.

Signed-off-by: Jessica Frazelle <jess@docker.com>
Merge pull request #3925 from jfrazelle/dockerfile

Add Dockerfile.