'Can\'t connect to MySQL server on \'db\' Django-Restframework with Mysql in docker

I have dockerized my existing Django Rest project which uses MySQL database.

Dockefile

FROM python:3.6

ENV PYTHONUNBUFFERED 1
RUN mkdir /code
WORKDIR /code
COPY . /code/
RUN pip install -r requirements.txt

requirements.txt

django
djongo
django-rest-framework
wheel
pillow
mysqlclient
django-cors-headers

docker-compose.yml

version: '3'
volumes:
    portainer:

services:
  db:
    image: mysql
    environment:
      MYSQL_ROOT_PASSWORD: docker
      MYSQL_DATABASE: docker
      MYSQL_USER: docker
      MYSQL_PASSWORD: docker
    ports:
      - "3306:3306"

  web:
    build: .
    command: bash -c "python manage.py makemigrations && python manage.py migrate && python manage.py runserver 0.0.0.0:8000 "
    volumes: 
      - .:/code
    ports:
      - "8000:8000"
    links:
     - db
  portainer:
   image: portainer/portainer
   ports:
   - "9000:9000"
   volumes:
    - /var/run/docker.sock:/var/run/docker.sock
    - portainer:/data

setting.py

        'ENGINE': 'django.db.backends.mysql', #django.db.backends.mysql 
        'NAME': 'docker', #local: libraries #server: 
        'USER': 'docker', #root #root
        'PASSWORD': 'docker', #local: root #server: 
        'HOST': 'db', #local: localhost  #server:
        'PORT': '3306',

My command 'docker-compose up -d --build' are successful but the restapp container was not started so while checking logs it was showing this error django.db.utils.OperationalError: (2003, 'Can\'t connect to MySQL server on \'db\' (111 "Connection refused")').