add nix compatible build environment

nixos/default.nix

@@ -0,0 +1,25 @@
+{
+  config,
+  pkgs,
+  lib,
+  ...
+}: let
+  cfg = config.services.wa2k;
+in {
+  imports = [./wa2k];
+
+  networking.firewall.allowedTCPPorts = lib.optional cfg.openFirewall [cfg.port];
+
+  # REF: https://nixos.wiki/wiki/Static_Web_Server
+  services.static-web-server = {
+    enable = true;
+    listen = "[::]:${cfg.port}";
+    root = "${pkgs.wa2k-website}/www";
+
+    configuration = {
+      general = {
+        directory-listing = false;
+      };
+    };
+  };
+}