Add 'shell.nix' for local development

As part of this I created a new files `./nixpkgs` that obtains a
specific version of the nixpkgs repo. This code was previously spread
between `release.nix` and `fetchNixpkgs.nix`. I also removed the Nix
1.11 code to simplify things. If we need Nix 1.11 support we can add it
again.

fetchNixpkgs.nix

@@ -1,50 +0,0 @@
-{ rev                             # The Git revision of nixpkgs to fetch
-, sha256                          # The SHA256 of the downloaded data
-, outputSha256 ? null             # The SHA256 fixed-output hash
-, system ? builtins.currentSystem # This is overridable if necessary
-}:
-
-if (0 <= builtins.compareVersions builtins.nixVersion "1.12")
-
-# In Nix 1.12, we can just give a `sha256` to `builtins.fetchTarball`.
-then (
-  builtins.fetchTarball {
-    url = "https://github.com/NixOS/nixpkgs/archive/${rev}.tar.gz";
-    sha256 = outputSha256;
-  })
-
-# This hack should at least work for Nix 1.11
-else (
-  (rec {
-    tarball = import <nix/fetchurl.nix> {
-      url = "https://github.com/NixOS/nixpkgs/archive/${rev}.tar.gz";
-      inherit sha256;
-    };
-
-    builtin-paths = import <nix/config.nix>;
-      
-    script = builtins.toFile "nixpkgs-unpacker" ''
-      "$coreutils/mkdir" "$out"
-      cd "$out"
-      "$gzip" --decompress < "$tarball" | "$tar" -x --strip-components=1
-    '';
-
-    nixpkgs = builtins.derivation ({
-      name = "nixpkgs-${builtins.substring 0 6 rev}";
-
-      builder = builtins.storePath builtin-paths.shell;
-
-      args = [ script ];
-
-      inherit tarball system;
-
-      tar       = builtins.storePath builtin-paths.tar;
-      gzip      = builtins.storePath builtin-paths.gzip;
-      coreutils = builtins.storePath builtin-paths.coreutils;
-    } // (if null == outputSha256 then { } else {
-      outputHashMode = "recursive";
-      outputHashAlgo = "sha256";
-      outputHash = outputSha256;
-    }));
-  }).nixpkgs)
-

nixpkgs.nix

@@ -0,0 +1,10 @@
+let
+  rev = "c5f9cd4cde81bcef2c8882d359d9d30313bebeb3";
+  outputSha256 = "1nilhz6rhfg3ckp8yfmgy6v3q6spbyxfg0yn8rc2ydyn7119h4fn";
+
+  nixpkgs = builtins.fetchTarball {
+    url = "https://github.com/NixOS/nixpkgs/archive/${rev}.tar.gz";
+    sha256 = outputSha256;
+  };
+in
+  import nixpkgs {}

release.nix

@@ -1,15 +1,5 @@
 let
-  fetchNixpkgs = import ./fetchNixpkgs.nix;
-
-  nixpkgs = fetchNixpkgs {
-    rev = "c5f9cd4cde81bcef2c8882d359d9d30313bebeb3";
-
-    sha256 = "12w6g50hgk5nmijgs3rv7kq5mdybnkyv3cr2zm0q3ka9ax180dk6";
-
-    outputSha256 = "1nilhz6rhfg3ckp8yfmgy6v3q6spbyxfg0yn8rc2ydyn7119h4fn";
-  };
-
-  pkgs = import nixpkgs {};
+  pkgs = import ./nixpkgs.nix;
 
   dhall-kubernetes-smoketests =
     pkgs.runCommand
@@ -42,4 +32,3 @@ in
       ];
     };
   }
-

shell.nix

@@ -0,0 +1,14 @@
+let
+  pkgs = import ./nixpkgs.nix;
+in
+  pkgs.stdenv.mkDerivation {
+    name = "dhall-kubernetes-shell";
+    buildInputs = [
+      pkgs.git
+      pkgs.dhall
+      pkgs.dhall-json
+      pkgs.python3
+      pkgs.python3Packages.requests
+      pkgs.glibcLocales
+    ];
+  }