Connect to mysql inside docker

I’m using the official MySQL image from docker hub and expose ports 3333:3306 to connect from outside.

I know that I have to change the bind IP inside /etc/mysql/my.cnf to the IP of this container and grant permission for a user like: GRANT ALL PRIVILEGES ON * . * TO 'newuser'@'container_ip'; so I can connect to this container by:

mysql -h container_ip -u root -p

But I received this error

ERROR 2002 (HY000): Can't connect to local MySQL server through socket '/var/run/mysqld/mysqld.sock' (2)

Btw, I also try to connect from WordPress in another container but it cannot establish the connection. Here is docker-compose.yml

version: '2'
services:
  mysqldb:
    image: mysql:5.6
    environment:
      MYSQL_ROOT_PASSWORD: password
    volumes:
      - ./mysql-data:/var/lib/mysql
      - ./mysql-import-data:/import-data
    ports:
      - "3333:3306"
  blog:
    image: webdevops/php-nginx:ubuntu-14.04
    environment:
      WEB_DOCUMENT_ROOT: /usr/share/nginx/html
    volumes:
      - ./blog:/usr/share/nginx/html
    ports:
      - "8080:80"
    depends_on:
      - mysqldb

1. What’s the mistake I made with this mysql container? I cannot connect to it.
2. The IP of the container may change every time docker-compose up. How can I configure it?