Without a resolver directive, nginx resolves upstream hostnames once at
startup and caches the IP forever. When the API container restarts it gets
a new Docker-assigned IP, causing 502 Bad Gateway until nginx is reloaded.
Fix:
- Add 'resolver 127.0.0.11 valid=10s' (Docker embedded DNS)
- Use a variable in proxy_pass ('set \ api') so nginx
re-resolves on every request using the resolver above
- Variable proxy_pass passes the full request URI as-is, so /api/...
correctly maps to http://api:8000/api/... without double-pathing
