git ignore stuff in sources for local builds

default.nix

@@ -2,10 +2,11 @@
 let
   sources = import nix/sources.nix;
   nixpkgs = import sources.nixpkgs { };
+  gitignore = import sources.gitignore { };
 in with nixpkgs;
 stdenv.mkDerivation {
   name = "elo-anything";
-  src = ./.;
+  src = gitignore.gitignoreSource ./.;
 
   buildInputs = [ elmPackages.elm ];
   buildPhase = ''

nix/sources.json

@@ -1,4 +1,16 @@
 {
+    "gitignore": {
+        "branch": "master",
+        "description": "Nix function for filtering local git sources",
+        "homepage": "",
+        "owner": "hercules-ci",
+        "repo": "gitignore",
+        "rev": "c4662e662462e7bf3c2a968483478a665d00e717",
+        "sha256": "1npnx0h6bd0d7ql93ka7azhj40zgjp815fw2r6smg8ch9p7mzdlx",
+        "type": "tarball",
+        "url": "https://github.com/hercules-ci/gitignore/archive/c4662e662462e7bf3c2a968483478a665d00e717.tar.gz",
+        "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
+    },
     "niv": {
         "branch": "master",
         "description": "Easy dependency management for Nix projects",