Add Dockerfile, which builds docker with nix package manager

With this Dockerfile, https://registry.hub.docker.com/u/offlinehacker/nixpkgs will automaticlly build docker image with nix package manager and nixpkgs(repo itself) on every push. To use this docker image: docker -t -i offlinehacker/nixpkgs. I advise nixos organization to create it's own account on docker registry and setup automated builds.

Example Dockefile that uses this image:

```
FROM offlinehacker/nixpkgs
RUN nix-env -iA nixpkgs.mongodb
RUN mkdir -p /var/db/mongodb
CMD mongod --dbpath /var/db/mongodb
```

PS: Docker is not a replacement for nixos, docker is process manager(which can also run whole nixos container), nixos is os, just to make this clear.