fix(config): use correct target in example proxy config

docker/proxy-config.example.json

@@ -1,6 +1,6 @@
 {
   "port": 5003,
-  "proxy_target": "http://localhost:5000",
+  "proxy_target": "http://backend:5000",
   "debug": true,
   "powered_by": "5 gay rats"
 }