DOCKER and WordPress: Error establishing a database connection

I am new to docker and i have this docker .yml file (below) and i have a local set up of my WordPress site, i have imported a sql dump into a database in the 'container_web'. The problem i am having is that when i try to connect WordPress to the database inside the container i get this error "Error establishing a database connection." For the host i've used localhost, 127.0.0.1, mysql and none of them seems to work.

Do you know or could direct me in the right direction on what i should be checking in order to connect my WordPress install to the database?

services:
  web:
    build: .
    container_name: 'container_web'
    dns: '8.8.8.8'
    volumes:
      - ./:/var/www/html
      - ./uploads.ini:/usr/local/etc/php/conf.d/uploads.ini
    depends_on:
      - db
    ports:
      - 80:80
    restart: always
    environment:
      WORDPRESS_ENV: 'development'

  db:
    container_name: 'container_db'
    image: mysql:5.7
    ports:
      - 3306:3306
    volumes:
      - ./:/var/lib/mysql
    restart: always
    environment:
      MYSQL_ROOT_PASSWORD: root
      MYSQL_DATABASE: mydatabase
      MYSQL_USER: user
      MYSQL_PASSWORD: password

volumes:
  mysql-data: {}