version: '2'
services:
  nginx:
    image: nginx
    container_name: nginx
    ports:
      - "80:80"
    volumes:
      - /etc/nginx/conf.d
    networks:
      - proxy-tier

  dockergen:
    image: jwilder/docker-gen
    command: -notify-sighup nginx -watch /etc/docker-gen/templates/nginx.tmpl /etc/nginx/conf.d/default.conf
    volumes_from:
      - nginx
    volumes:
      - /var/run/docker.sock:/tmp/docker.sock:ro
      - ./nginx.tmpl:/etc/docker-gen/templates/nginx.tmpl
    networks:
      - proxy-tier

  whoami:
    image: jwilder/whoami
    environment:
      - VIRTUAL_HOST=whoami.local
    networks:
      - proxy-tier

networks:
  proxy-tier:
    driver: bridge

version: '2'
services:
  nginx:
    image: nginx
    container_name: nginx
    ports:
      - "80:80"
    volumes:
      - /etc/nginx/conf.d

  dockergen:
    image: jwilder/docker-gen
    command: -notify-sighup nginx -watch /etc/docker-gen/templates/nginx.tmpl /etc/nginx/conf.d/default.conf
    volumes_from:
      - nginx
    volumes:
      - /var/run/docker.sock:/tmp/docker.sock:ro
      - ./nginx.tmpl:/etc/docker-gen/templates/nginx.tmpl

  whoami:
    image: jwilder/whoami
    environment:
      - VIRTUAL_HOST=whoami.local

Remove proxy-tier network in favor of the default.
Remove proxy-tier network in favor of the default.

As @huiwang pointed out, using a custom network is unnecessary since the
default bridge network works just as well.
