fix: environment variable API_URL can't be changed

reverse-proxy/Caddyfile

@@ -1,7 +1,7 @@
 :3000 {
     # Reverse proxy for /api
-    reverse_proxy /api/* http://localhost:8080
+    reverse_proxy /api/* http://localhost:{$BACKEND_PORT:8080}
 
     # Reverse proxy for all other requests
-    reverse_proxy http://localhost:3333
+    reverse_proxy http://localhost:{$PORT:3333}
 }