Gemfile

source 'http://rubygems.org'

gem 'nokogiri'

Gemfile.lock

GEM
  remote: http://rubygems.org/
  specs:
    mini_portile2 (2.0.0)
    nokogiri (1.6.7.2)
      mini_portile2 (~> 2.0.0.rc2)

PLATFORMS
  ruby

DEPENDENCIES
  nokogiri

BUNDLED WITH
   1.11.2

gemset.nix

{
  mini_portile2 = {
    source = {
      remotes = ["http://rubygems.org"];
      sha256 = "056drbn5m4khdxly1asmiik14nyllswr6sh3wallvsywwdiryz8l";
      type = "gem";
    };
    version = "2.0.0";
  };
  nokogiri = {
    dependencies = ["mini_portile2"];
    source = {
      remotes = ["http://rubygems.org"];
      sha256 = "11sbmpy60ynak6s3794q32lc99hs448msjy8rkp84ay7mq7zqspv";
      type = "gem";
    };
    version = "1.6.7.2";
  };
}

shell.nix

let
  nixpkgs = (import <nixpkgs> {}).pkgs.fetchgit {
    url = "https://github.com/nixos/nixpkgs-channels";
    rev = "7465bcd67a2177c63adeec66398b1b581260b31e";
    sha256 = "08ffc5i6kzgf7d5q8wcr8q6vpjvq3i95vhvg482k34k1m3v1p7f9";
  };
in
with (import nixpkgs {});
let
  ruby = ruby_2_3;
  env = bundlerEnv {
    name = "foo";
    inherit ruby;
    gemset = ./gemset.nix;
    gemfile = ./Gemfile;
    lockfile = ./Gemfile.lock;
    groups = ["default" "development" "test" "production"];
  };
in stdenv.mkDerivation rec {
  name = "foo";
  buildInputs = [
    bashInteractive
    env
    env.ruby
    env.bundler
  ];
}