Ensure MySQL is ready without some dirty bash script trying to connect

docker-compose.yml

version: "2.1"
services:
  app:
    build:
      dockerfile: ./build/docker/app/Dockerfile
      context: .
    env_file:
      - ./build/docker/app/app.env
      - ./build/docker/mysql/mysql.env
    extra_hosts:
      - app.docker:127.0.0.1
    ports:
      - 80:80
    restart: always
    depends_on:
      db:
        condition: service_healthy
        
  db:
    build:
      dockerfile: ./build/docker/mysql/Dockerfile
      context: .
    env_file:
      - ./build/docker/mysql/mysql.env
    ports:
      - 3306:3306
    healthcheck:
      test: ["CMD", "nc", "-z", "localhost", "3306"]
      interval: 2s
      timeout: 3s
      retries: 15

Dockerfile

FROM mysql:5.7.16

RUN apt-get update \
    && apt install -y netcat \
    && apt-get clean && apt-get purge \
    && rm -rf /var/lib/apt/lists/* /var/cache/apt/*