Add Nix package

[nightkr]

This is largely copied from https://github.com/stackabletech/beku.py/blob/f5ef0199b0c3c8aa277a275a9d3c624eb5804f27/beku.nix

[sbernauer]

Many thanks!

Tested using

  image-tools = pkgs.callPackage(pkgs.fetchFromGitHub {
    owner = "stackabletech";
    repo = "image-tools";
    rev = "feature/nix";
    hash = "sha256-gjTCroHw4iJhXPW+s3mHBzIH8seIKH1tPb82lUb8+a0="; # use lib.fakeHash to find new hashes when upgrading
  } + "/image-tools.nix") {};

Do we want to add this example to the README as well, similar to beku.py?

[nightkr]

Sure.

[sbernauer]

Awesome, thanks!