mysql:
  restart: always
  image: mysql:5.7
  container_name: mysql
  environment:
    - MYSQL_ROOT_PASSWORD=12345
    - MYSQL_DATABASE=abc
    - MYSQL_USER=abc
    - MYSQL_PASSWORD=abc
nginx:
  restart: always
  image: phpdockerio/nginx:latest
  container_name: nginx
  volumes:
      - ..:/www/example.com
      - ./nginx/nginx.conf:/etc/nginx/conf.d/default.conf
  ports:
   - "80:80"
  links:
   - php-fpm

php-fpm:
  restart: always
  build: .
  dockerfile: php-fpm/Dockerfile
  container_name: php-fpm
  volumes:
    - ..:/www/example.com
    - ./php-fpm/php-ini-overrides.ini:/etc/php/7.1/fpm/conf.d/99-overrides.ini
  links:
    - mysql

[Note] Server hostname (bind-address): '*'; port: 3306
[Note] IPv6 is available.
[Note]   - '::' resolves to '::';
[Note] Server socket created on IP: '::'.
mysql      | 2017-01-14T09:53:06.345013Z 0 [Note] mysqld: ready for connections.
mysql      | Version: '5.7.17'  socket: '/var/run/mysqld/mysqld.sock'  port: 3306  MySQL Community Server (GPL)

return [
    'class' => 'yiidbConnection',
    'dsn' => 'mysql:host=127.0.0.1;dbname=abc',
    'username' => 'root',
    'password' => '12345',
    'charset' => 'utf8',
];

$ret = Yii::$app->db->createCommand('SHOW VARIABLES')->queryAll();
print_r($ret);

Database Exception – yiidbException
SQLSTATE[HY000] [2002] Connection refused