add fixed-output derivation

fixed.nix

+{ nixpkgs ? import <nixpkgs> {} }:
+with nixpkgs;
+
+stdenv.mkDerivation {
+  name = "walint-fixed";
+
+  buildInputs = [ ghc stack zlib zlib.dev git openssl cacert ];
+
+  src = ./.;
+
+  buildPhase = ''
+    cp -r $src .
+    mkdir /tmp/stack-home
+    HOME=/tmp/stack-home stack build --no-nix --system-ghc
+  '';
+
+  installPhase = ''
+    HOME=/tmp/stack-home stack install --local-bin-path $out --no-nix --system-ghc
+    mkdir -p $out/share/walint
+    cp -r static $out/share/walint
+    cp config.json $out/share/walint
+    cp config.toml $out/share/walint
+  '';
+
+  outputHashAlgo = "sha256";
+  outputHashMode = "recursive";
+  # replace this with the correct SHA256
+  outputHash = "sha256-Qd7MDGslrS6zs6WWI9sjzDous0nUbrjdK2fF747KLq8=";
+  dontShrink = true;
+}