'db'의 MySQL 서버에 연결할 수 없습니다 ([Errno -2] 이름이 확인되지 않음)

파이썬을 사용하여 mysql을 docker와 연결하려고합니다. 그러나이 오류가 발생합니다. python3을 사용하고 있습니다. 나는 도커를 처음 사용하므로 해결책을 찾을 수 없습니다.

Dockerfile :

FROM python:3.8-alpine
MAINTAINER Fahim Ahmed Irfan

ENV PYTHONUNBUFFERED 1

COPY ./requirements.txt /requirements.txt
RUN apk add --update --no-cache jpeg-dev
RUN apk --update add --no-cache --virtual .tmp-build-deps \
      gcc build-base libffi-dev freetype-dev libpng-dev openblas-dev linux-headers mysql-dev zlib zlib-dev
RUN pip install -r /requirements.txt

RUN mkdir /app
WORKDIR ./app
COPY ./app /app

RUN adduser -D user
User user
EXPOSE 8000

Docker 작성 파일 :

version: '3'

services:
  app:
    restart: always
    build:
      context: .
    ports:
      - "8000:8000"
    volumes:
      - ./app:/app
    command: >
      sh -c "python check_db.py --service-name mysql --ip db --port 3306 &&
             python manage.py makemigrations &&
             python manage.py migrate &&
             python manage.py runserver 0.0.0.0:8000"
    environment:
      - DB_HOST=db
      - DB_NAME=app
      - DB_USER=root
      - DB_PASS=supersecretpassword
    depends_on:
      - db

  db:
    image: mysql:8.0
    environment:
      - MYSQL_HOST=localhost
      - MYSQL_PORT=3306
      - MYSQL_ROOT_HOST=%
      - MYSQL_DATABASE=app
      - MYSQL_USER=root
      - MYSQL_PASSWORD=supersecretpassword

    ports:
      - "3306:3306"

check_db.py :

import socket
import time
import argparse
""" Check if port is open, avoid docker-compose race condition """

parser = argparse.ArgumentParser(description='Check if port is open, avoid\
                                 docker-compose race condition')
parser.add_argument('--service-name', required=True)
parser.add_argument('--ip', required=True)
parser.add_argument('--port', required=True)

args = parser.parse_args()

# Get arguments
service_name = str(args.service_name)
port = int(args.port)
ip = str(args.ip)

# Infinite loop
while True:
    sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    result = sock.connect_ex((ip, port))
    if result == 0:
        print("{0} port is open! Bye!".format(service_name))
        break
    else:
        print("{0} port is not open! I'll check it soon!".format(service_name))
        time.sleep(3)

settings.py :

DATABASES = {
    'default': {
        'ENGINE': 'django.db.backends.mysql',
        'HOST': os.environ.get('DB_HOST'),
        'NAME': os.environ.get('DB_NAME'),
        'USER': os.environ.get('DB_USER'),
        'PASSWORD': os.environ.get('DB_PASS'),
        'PORT': '3306',
    }

requirements.txt :

Django==2.1.3,<2.2.0
djangorestframework==3.9.0,<3.10.0
pymysql

오류 : django.db.utils.OperationalError : (2003, "Ca n't connect to MySQL server on 'db'([Errno -2] Name does not resolve)")

이 문제를 해결하는 방법을 모릅니다. 조언 부탁드립니다.