nix-docker bootstrap and start script

[adelbertc]

So I may have gone overboard and rewritten the bootstrap script into something fancier. Unsure if this is something you'd want as part of the project but I'm finding it useful to have on my MacBook to build Docker images with since Nix + Docker + OSX does not make for a good time.

Also per our IRC conversation earlier today, the missing pieces were:

• The README's suggestion of nix-build -A hello --argstr system x86_64-linux to test seems to no longer work for reasons beyond me. I ended up testing with the following derivation:

with import <nixpkgs> { system = "x86_64-linux"; };

pkgs.dockerTools.buildImage {
  name = "nix-htop";
  contents = pkgs.htop;
  config = {
    Cmd = [ "/bin/htop" ];
  };
}

• I tried using the original script but that was missing setting up of the environment variables done in remote-build-env.