add nix shell file

shell.nix

@@ -0,0 +1,31 @@
+let
+  pkgs = import <nixpkgs> {};
+  stdenv = pkgs.stdenv;
+  packer = stdenv.mkDerivation {
+    name = "packer";
+    src = pkgs.fetchurl {
+      url = "https://dl.bintray.com/mitchellh/packer/0.6.1_linux_amd64.zip";
+      sha256 = "1sjsp0jqmhc9frp6hm1g0zd42dipxwbd9xanhnf504yvnxdaa4ww";
+    };
+    buildInputs = [
+      pkgs.unzip
+    ];
+    phases = [ "installPhase" ];
+    installPhase =
+      ''
+      ensureDir $out/bin
+      unzip $src -d $out/bin
+      '';
+  };
+in 
+stdenv.mkDerivation rec {
+  name = "shell";
+  version = "shell";
+  src = ./.;
+  buildInputs = [ 
+    packer
+  ];
+  shellHook = ''
+    export GOPATH=$(pwd)
+    '';
+}