For a more mature project that solves the same problem have a look here: https://github.com/siers/nix-gitignore

nix-git-ignore-source

Use a .gitignore file to filter out source files from nix derivations.

API

This expression has two attributes gitIgnoreSource and gitIgnoreFilter.

gitIgnoreSource just takes one argument which is a path to a directory with a .gitignore file in it.

gitIgnoreFilter is a more general function that plugs into builtins.filterSource by taking first a path to a .gitignore then the same two parameters as given by filterSource, the path and type of the file to check if it is ignored by the ignore file.

Example usage

{
  pkgs ? import <nixpkgs> {},
  gis ? import (fetchTarball {
    url = https://github.com/icetan/nix-git-ignore-source/archive/v1.0.0.tar.gz;
    sha256 = "1mnpab6x0bnshpp0acddylpa3dslhzd2m1kk3n0k23jqf9ddz57k";
  }) {},
}:

pkgs.stdenv.mkDerivation {
  name = "example";
  src = gis.gitIgnoreSource ./.;
}