I have nginx as a proxy for a python application. To launch them I have a docker-compose.

services:
  app:
    ...
  nginx:
    ...
    ports:
      - "80:80"
      - "443:443"

server {
    ...

    location / {
        proxy_pass http://app:8080;
        proxy_set_header Host $host;
        proxy_set_header X-Real-IP $remote_addr;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
    }
}

With this configuration I got two problems.

Do not name the service in docker-compose.yml as app

In nginx:alpine it resolves as 127.0.53.53. And if nginx is started before an app, it will always return 502 error. So I renamed it to backend.

The next error - [emerg] host not found in upstream

Now nginx fails when it’s starting before the backend, but it fails fast and it’s better. To solve this in local links or depends_on can be used.

In swarm mode these options are not supported, but it’s not a problem, because with a restart_policy it just continues to restart until the backend is resolvable.