This is my attempt to solve the same nginx-proxy#205 issue.

In Swarm Mode and Docker Cloud, container names are dynamic. nginx-proxy#181 solved this partially with a label on jwilder/nginx-proxy but not if they're separate containers. nginx-proxy#126 tries a different approach, but I think using a label is a better one.

This PR simply adds a com.github.jrcs.letsencrypt_nginx_proxy_companion.docker_gen=true label to the docker-gen container.

It also moves the label related detection to runtime (functions.sh: nginx-reload) instead of startup time (entrypoint.sh), because the nginx and docker-gen containers could be redeployed during the lifecycle of the letsencrypt companion, and thus, getting different ids.

There's also a new assumption in that I think it's simpler to just recommend using the label instead of volumes-from to detect the nginx-proxy container. The reason is that in some environments (like mine), you can't use volumes-from, but I think you can always use labels.

Note: If you're wandering why I've chosen not to set the variables NGINX_DOCKER_GEN_CONTAINER and NGINX_PROXY_CONTAINER, that's because we can use them to make sure an explicitly set container name takes precedence over a label.

Now I'm running a successful separate containers deployment in Swarm Mode, here's my stack:

version: '3.2'
services:
  nginx:
    image: nginx:alpine
    ports:
      - 80:80
      - 443:443
    volumes:
      - ./nginx/conf.d:/etc/nginx/conf.d
      - ./nginx/vhost.d:/etc/nginx/vhost.d
      - ./nginx/certs:/etc/nginx/certs
      - ./nginx/letsencrypt:/usr/share/nginx/html
    labels:
      - com.github.jrcs.letsencrypt_nginx_proxy_companion.nginx_proxy=true
    deploy:
      placement:
        constraints:
          - node.role == manager

  dockergen:
    image: helder/docker-gen
    command: -notify "docker-label-sighup com.github.jrcs.letsencrypt_nginx_proxy_companion.nginx_proxy" -watch /etc/docker-gen/templates/nginx.tmpl /etc/nginx/conf.d/default.conf
    volumes:
      - /var/run/docker.sock:/tmp/docker.sock:ro
      - ./nginx.tmpl:/etc/docker-gen/templates/nginx.tmpl
      - ./nginx/conf.d:/etc/nginx/conf.d
      - ./nginx/vhost.d:/etc/nginx/vhost.d
      - ./nginx/certs:/etc/nginx/certs
    labels:
      - com.github.jrcs.letsencrypt_nginx_proxy_companion.docker_gen=true
    deploy:
      placement:
        constraints:
          - node.role == manager

  letsencrypt:
    image: helder/letsencrypt-nginx-proxy-companion:latest
    volumes:
      - /var/run/docker.sock:/var/run/docker.sock:ro
      - ./nginx/conf.d:/etc/nginx/conf.d
      - ./nginx/vhost.d:/etc/nginx/vhost.d
      - ./nginx/certs:/etc/nginx/certs
      - ./nginx/letsencrypt:/usr/share/nginx/html
    deploy:
      placement:
        constraints:
          - node.role == manager

As you can see, I'm also using a custom helder/docker-gen image which simply adds a docker-label-sighup script that allows me to reload the nginx container from a label, because... well, dynamic names. jwilder/docker-gen doesn't support reloading containers with dynamic names (yet).