I have a Docker composition of an Express application and MySQL database.
When I run docker-compose up –build, everything works perfectly.
However, if I try to run the created image separately, the MySQL server is not recognized.

Here is my docker-compose.yml file:

version: '3.8'

services:
  mysql:
    image: mysql
    container_name: mysql-for-compose
    environment:
      MYSQL_ROOT_PASSWORD: 'password'
      MYSQL_DATABASE: 'testdb'
    volumes:
      - mysql-data:/var/lib/mysql
    ports:
      - "3306:3306"
    networks:
      - express-mysql-network

  express:
    build: .
    container_name: express-for-compose
    ports:
      - "4000:4000"
    networks:
      - express-mysql-network
    depends_on:
      - mysql

networks:
  express-mysql-network:

volumes:
  mysql-data:

When I run the following command:

docker-compose up –build

Everything works perfect.

But when I run afterward the created image:
docker run -p 4000:4000 –network express-mysql-network

If the app tries to talk to MySQL, it fails with the following error:

Error: getaddrinfo EAI_AGAIN mysql
    at GetAddrInfoReqWrap.onlookup [as oncomplete] (node:dns:109:26) {
    errno: -3001,
    code: 'EAI_AGAIN',
    syscall: 'getaddrinfo',
    hostname: 'mysql',
    fatal: true
}

Can you help explain why?