How to properly configure an Apache reverse proxy for multiple domains? or Apache reverse proxy setup

504s usually mean the proxy can’t reach the backend. Double-check:

- Is `backend:8080` accessible from the apache server itself? Try `curl http://backend:8080` on the proxy box.
- Maybe SELinux or AppArmor blocking the connection? Temporarily disable to test.

Btw, for debugging, enable apache reverse proxy logs:
```
LogLevel debug
```