Fix: Nginx 502 Bad Gateway Error

Nginx returns 502 when it can't get a valid response from the upstream server. The upstream is either not running, on the wrong port, or unreachable on the configured address.

Diagnosis checklist

# 1. Is the upstream service running?
docker ps | grep your-service
systemctl status your-service

# 2. Is it listening on the expected port?
ss -tlnp | grep 3000

# 3. Can Nginx reach it?
curl -sI http://127.0.0.1:3000/

# 4. Check Nginx error log:
tail -50 /var/log/nginx/error.log | grep upstream