Minor fixes.

flake.nix

@@ -30,15 +30,28 @@
           mkdir -p $out
           cp -r . $out/
         '';
-        postFixup = ''
-          substituteInPlace $out/bin/infinsweeper \
-            --replace-fail '#!/usr/bin/env ruby' '#!${ruby}/bin/ruby'
+      };
+
+      server = pkgs.writeShellApplication {
+        name = "infinsweeper";
+        runtimeInputs = [ ruby ];
+        text = ''
+          set -euo pipefail
+          data_dir="''${INFINSWEEPER_DATA_DIR:-/var/lib/infinsweeper}"
+          DB="$data_dir/infinsweeper.db"
+          mkdir -p "$data_dir"
+          exec ruby ${app}/main.rb
         '';
       };
     in
     {
       packages.${system}.default = app;
 
+      apps.${system}.default = {
+        type = "app";
+        program = "${server}/bin/infinsweeper";
+      };
+
       devShells.${system}.default = pkgs.mkShell {
         packages = [
           ruby