How to setup Django with mysql on Docker?

I am new to DevOps and I am trying to Dockerize my Django-Mysql connection.My Docker file looks like this:-

FROM python:3.6
ENV PYTHONUNBUFFERED 1

RUN mkdir /docker_dir

WORKDIR /docker_dir

ADD . /docker_dir/
RUN pip install -r requirements.txt

My docker-compose.yml file looks like this:-

version: '3'

services:
  db:
    image: mysql
    restart: always
    command: --default-authentication-plugin=mysql_native_password --mysqlx=0
    environment:
      - MYSQL_HOST=127.0.0.1
      - MYSQL_PORT=3306  # cannot change this port to other number
      - MYSQL_DATABASE=cgi_assignments # name you want for the database
      - MYSQL_USER="root" # change to whatever username you want
      - MYSQL_PASSWORD="root" #change to the password you want for user
      - MYSQL_ROOT_PASSWORD="root" #change to good root password
    ports:
      - "3307:3306"
    volumes:
      - .setup.sql:/docker-entrypoint-initbd.d/setup.sql
  
  web:
    build: .
      # command: python manage.py runserver 0.0.0.0:8000
    command: python manage.py runserver 0.0.0.0:8000
    container_name: docker_dir
    volumes:
      - .:/docker_dir
    ports:
      - "8000:8000"
    depends_on:
      - db
    links: 
      - db 

and my database on Django settings file looks like this:-

DATABASES = {
    'default': {
        'NAME': 'cgi_assignments',
        'ENGINE': 'django.db.backends.mysql',
        'USER': 'root',
        'PASSWORD': 'root',
        'HOST': 'db',
        'PORT': 3306,
    },

}

If I use ports:-"3307:3306" it gives me the following error:-

django.db.utils.OperationalError: (1130, "Host '172.18.0.3' is not allowed to connect to this MySQL server")

and I use ports:-"3306:3306" it gives me the following error:-

 listen tcp 0.0.0.0:3306: bind: address already in use