db:
  image: mysql
  environment:
    MYSQL_ROOT_PASSWORD: <%= ENV['MYSQL_ROOT_PASSWORD'] %>
  ports:
    - "3306:3306"

web:
  build: .
  command: bundle exec rails s -p 3000 -b '0.0.0.0'
  volumes:
    - .:/myapp
  ports:
    - "3000:3000"
  links:
    - db

default: &default
  adapter: mysql2
  encoding: utf8
  pool: 5
  username: root
  password: <%= ENV['MYSQL_ROOT_PASSWORD'] %>
  host: <%= ENV['DB_PORT_3306_TCP_ADDR'] %>
  port: <%= ENV['DB_PORT_3306_TCP_PORT'] %>

development:
  <<: *default
  database: myapp_development

test:
  <<: *default
  database: myapp_test

production:
  <<: *default
  database: myapp_production

$ docker-compose up
...
sidekiq_1 | Host '172.17.0.5' is not allowed to connect to this MySQL server
...

$ docker-compose run web env
...
DB_PORT=tcp://172.17.0.3:3306
DB_PORT_3306_TCP=tcp://172.17.0.3:3306
DB_PORT_3306_TCP_ADDR=172.17.0.3
DB_PORT_3306_TCP_PORT=3306
DB_PORT_3306_TCP_PROTO=tcp
WEB_PORT=tcp://172.17.0.6:3000
WEB_PORT_3000_TCP=tcp://172.17.0.6:3000
WEB_PORT_3000_TCP_ADDR=172.17.0.6
WEB_PORT_3000_TCP_PORT=3000
WEB_PORT_3000_TCP_PROTO=tcp
...

$ docker-machine ip myapp
192.168.99.100

Mysql2::Error
Host '172.17.0.6' is not allowed to connect to this MySQL server

default: &default
  adapter: mysql2
  encoding: utf8
  pool: 5
  username: root
  password: <%= ENV['MYSQL_ROOT_PASSWORD'] %>
  host: db
development:
  <<: *default
  database: myapp_development

test:
  <<: *default
  database: myapp_test

production:
  <<: *default
  database: myapp_production

default: &default
  adapter: mysql2
  encoding: utf8
  pool: 5
  username: root
  password:
  host: 127.0.0.1