Simplify the build, especially the final layer count

- since we're already throwing so much into `make_iso.sh`, and it's useful and easy to manage there, push more in to lower our layer count
- replace `ADD` with `COPY` as appropriate
- update our AUFS `git clone` to be much more efficient by using `-b ...` and `--depth 1` instead of just `git clone ... && git checkout ...`
- update our "curl docker" to curl the version of Docker that corresponds to our boot2docker version
- move `rootfs/rootfs/etc/version` to a top-level `VERSION` now that our `Dockerfile` is also top-level, both so it's more discoverable for people checking out our repo, and so it's easier to find and update at release-time for maintainers
- update "git version info" to only copy in `.git` so it's slightly smaller and simpler

Of course, if any of this is controversial, I'm happy to split it out, but it all seemed so nice together that I figured I'd throw it all out to get thoughts.