Changed to dockerfile.vim included in Docker.

- Added syntax for specifying subtree location.
  - Add subtree location as a fourth colon separated value in PACKS e.g. `dockerfile:docker/docker::/contrib/syntax/vim/`
- Fetching dockerfile.vim from docker project.
- **NOTE** This might be on its way to Vim proper:
  https://groups.google.com/forum/#!topic/vim_dev/70768C8wxf4