Example Docker Compose file that links WordPress container to an already running MySQL container using external links.

docker-compose.yml

version: '3'

services:
  wordpress:
    image: wordpress:4.9.4-php7.0-apache
    network_mode: bridge
    external_links:
      - db-mysql:db
    volumes:
      - "./wp-content/themes:/var/www/html/wp-content/themes"
      - "./wp-content/plugins:/var/www/html/wp-content/plugins"
      - "./wp-content/uploads:/var/www/html/wp-content/uploads"
      - "./.htaccess:/var/www/html/.htaccess"
    ports:
      - "8080:80"
    restart: always
    container_name: wordpress-dev
    environment:
      WORDPRESS_DB_HOST: db:3306
      WORDPRESS_DB_NAME: wordpress_dev
      WORDPRESS_DB_USER: someusername
      WORDPRESS_DB_PASSWORD: somepassword

Thank you! The gist is really helpful.