Add optional apt-cacher proxy configuration.

Support APT_PROXY build arg in Docker/Compose, document setup, and include a safe .env.example template for local cache usage.

Co-authored-by: Cursor <cursoragent@cursor.com>

docker-compose.yml

@@ -1,5 +1,8 @@
 x-game-template: &game-template
-  build: .
+  build:
+    context: .
+    args:
+      APT_PROXY: ${APT_PROXY:-}
   user: "root"
   restart: unless-stopped
   privileged: true