docker-gen with dynamic container names support

This is an enhancement to the docker-gen image that adds a script that can send a SIGHUP signal to a container through the use of a label.

The problem

The usual way of using docker-gen in conjunction with docker-letsencrypt-nginx-proxy-companion using the separate container method, is as follows (as per the docs):

$ docker run -d \
    --name nginx-gen \
    --volumes-from nginx \
    -v /path/to/nginx.tmpl:/etc/docker-gen/templates/nginx.tmpl:ro \
    -v /var/run/docker.sock:/tmp/docker.sock:ro \
    jwilder/docker-gen \
    -notify-sighup nginx -watch -only-exposed -wait 5s:30s /etc/docker-gen/templates/nginx.tmpl /etc/nginx/conf.d/default.conf

However, within a Docker Cloud or Swarm Mode based environment we cannot use -notify-sighup nginx due to the fact that the container names (on the actual nodes) do not match their service names. The result is that the nginx container (Service) never get's reloaded to take advantage of the generated Nginx configuration.

The solution

Add a label to the container you want to reload, with value true. Then, use the bundled docker-label-sighup script, with the label name as an argument. For example:

services:
  nginx:
    image: nginx:alpine
    ports:
      - 80:80
      - 433:433
    volumes:
      - nginx:/etc/nginx/conf.d
      - nginx:/etc/nginx/certs
    labels:
      - com.example.nginx_proxy=true

  dockergen:
    image: ghcr.io/helderco/docker-gen:master
    command: -notify "docker-label-sighup com.example.nginx_proxy" -watch -only-exposed -wait 10s:30s /etc/docker-gen/templates/nginx.tmpl /etc/nginx/conf.d/default.conf
    volumes:
      - nginx:/etc/nginx/conf.d
      - /var/run/docker.sock:/tmp/docker.sock:ro
      - ./nginx.tmpl:/etc/docker-gen/templates/nginx.tmpl

volumes:
  nginx:

Note the use of -notify instead of using -notify-sighup to redeploy the service using the bundled script.