$ docker ps
CONTAINER ID        IMAGE               COMMAND                  CREATED             STATUS              PORTS               NAMES
4eea64fee19e        mariadb:10.1.11     "/docker-entrypoint.s"   27 minutes ago      Up 10 minutes       3306/tcp            db1_db_1

-----

$ cat docker-compose.yml
  version: '2'
  services:
    web:
      build:
        context: .
        dockerfile: Dockerfile-www
      external_links:
        - "db1_db_1:mysql"
      ports:
        - 80:80
      environment:
        WORDPRESS_DB_HOST: mysql
        WORDPRESS_DB_USER: l_wp
        WORDPRESS_DB_PASSWORD: blahblah
        WORDPRESS_DB_NAME: www