support for systemd-notify mode + nix dependencies build by bundix

Gemfile

@@ -4,3 +4,4 @@ gem 'net-dns'
 gem 'prometheus-client'
 gem 'rack'
 gem 'puma'
+gem 'sd_notify'

gemset.nix

@@ -0,0 +1,63 @@
+{
+  net-dns = {
+    groups = ["default"];
+    platforms = [];
+    source = {
+      remotes = ["https://rubygems.org"];
+      sha256 = "18d97xjphw21naaqfhgxp95ikr1d79rx708b2df3xm01j6isqy1d";
+      type = "gem";
+    };
+    version = "0.9.0";
+  };
+  nio4r = {
+    groups = ["default"];
+    platforms = [];
+    source = {
+      remotes = ["https://rubygems.org"];
+      sha256 = "0xk64wghkscs6bv2n22853k2nh39d131c6rfpnlw12mbjnnv9v1v";
+      type = "gem";
+    };
+    version = "2.5.8";
+  };
+  prometheus-client = {
+    groups = ["default"];
+    platforms = [];
+    source = {
+      remotes = ["https://rubygems.org"];
+      sha256 = "11k1r8mfr0bnd574yy08wmpzbgq8yqw3shx7fn5f6hlmayacc4bh";
+      type = "gem";
+    };
+    version = "4.0.0";
+  };
+  puma = {
+    dependencies = ["nio4r"];
+    groups = ["default"];
+    platforms = [];
+    source = {
+      remotes = ["https://rubygems.org"];
+      sha256 = "0dgr2rybayih2naz3658mbzqwfrg9fxl80zsvhscf6b972kp3jdw";
+      type = "gem";
+    };
+    version = "5.6.4";
+  };
+  rack = {
+    groups = ["default"];
+    platforms = [];
+    source = {
+      remotes = ["https://rubygems.org"];
+      sha256 = "0i5vs0dph9i5jn8dfc6aqd6njcafmb20rwqngrf759c9cvmyff16";
+      type = "gem";
+    };
+    version = "2.2.3";
+  };
+  sd_notify = {
+    groups = ["default"];
+    platforms = [];
+    source = {
+      remotes = ["https://rubygems.org"];
+      sha256 = "0c9imnjbakx25r2n7widfp00s19ndzmmwax761mx5vbwm9nariyb";
+      type = "gem";
+    };
+    version = "0.1.1";
+  };
+}

shell.nix

@@ -0,0 +1,13 @@
+with (import <nixpkgs> {});
+let
+  env = bundlerEnv {
+    name = "dnsbl_exporter-bundler-env";
+    inherit ruby;
+    gemfile  = ./Gemfile;
+    lockfile = ./Gemfile.lock;
+    gemset   = ./gemset.nix;
+  };
+in stdenv.mkDerivation {
+  name = "dnsbl_exporter";
+  buildInputs = [ pkgs.ruby env env.gems.puma ];
+}