Gitlab Ci管道问题：Node.js无法连接到Mysql数据库

我正在尝试建立一个Gitlab Ci管道，可以在其中测试我的nodejs应用程序。问题是，该应用程序无法连接到数据库。我试图像这里配置一切。我真的不知道为什么它找不到数据库，也找不到任何其他示例或资源。问题是，该应用程序需要数据库运行，否则它将停止。

对于使用typeorm的数据库连接即时消息：

"type": "mysql",
   "host": "db",
   "port": 3306,
   "username": "root",
   "password": "logistics",
   "database": "logistics",
   "synchronize": true,
   "logging": true,
   "entities": [
      "src/entity/**/*.ts"
   ],
   "migrations": [
      "src/migration/**/*.ts"
   ],
   "subscribers": [
      "src/subscriber/**/*.ts"
   ],
   "cli": {
      "entitiesDir": "src/entity",
      "migrationsDir": "src/migration",
      "subscribersDir": "src/subscriber"
   }

这是我的gitlab.yml：

# https://hub.docker.com/r/library/node/tags/
image: node:13

variables:
  # Configure mysql environment variables (https://hub.docker.com/_/mysql/)
  MYSQL_DATABASE: "logistics"
  MYSQL_ROOT_PASSWORD: "logistics"
  MYSQL_USER: "paramedic"
  MYSQL_PASSWORD: "logistics"
  MYSQL_PORT: "3306"
  
# Check out: http://docs.gitlab.com/ce/ci/docker/using_docker_images.html#what-is-a-service
services:
  - name: mysql:5.7
  
# http://docs.gitlab.com/ce/ci/yaml/README.html#cache
cache:
  paths:
    - backend/node_modules/

test_backend:
  script:
    - cd backend/
    - yarn install
    - yarn start

这是错误消息：

$ yarn start
 yarn run v1.22.4
 warning package.json: No license field
 $ ts-node-dev --project=tsconfig.json --transpileOnly --respawn src/index.ts
 Using ts-node version 8.10.2, typescript version 3.9.2
 Error: getaddrinfo ENOTFOUND db
     at GetAddrInfoReqWrap.onlookup [as oncomplete] (dns.js:66:26)
     --------------------
     at Protocol._enqueue (/builds/stjorueh/rettungsdienstverwalter/backend/node_modules/mysql/lib/protocol/Protocol.js:144:48)
     at Protocol.handshake (/builds/stjorueh/rettungsdienstverwalter/backend/node_modules/mysql/lib/protocol/Protocol.js:51:23)
     at PoolConnection.connect (/builds/stjorueh/rettungsdienstverwalter/backend/node_modules/mysql/lib/Connection.js:116:18)
     at Pool.getConnection (/builds/stjorueh/rettungsdienstverwalter/backend/node_modules/mysql/lib/Pool.js:48:16)
     at /builds/stjorueh/rettungsdienstverwalter/backend/src/driver/mysql/MysqlDriver.ts:884:18
     at new Promise (<anonymous>)
     at MysqlDriver.createPool (/builds/stjorueh/rettungsdienstverwalter/backend/src/driver/mysql/MysqlDriver.ts:881:16)
     at MysqlDriver.<anonymous> (/builds/stjorueh/rettungsdienstverwalter/backend/src/driver/mysql/MysqlDriver.ts:343:36)
     at step (/builds/stjorueh/rettungsdienstverwalter/backend/node_modules/tslib/tslib.js:139:27)
     at Object.next (/builds/stjorueh/rettungsdienstverwalter/backend/node_modules/tslib/tslib.js:120:57) {
   errno: -3008,
   code: 'ENOTFOUND',
   syscall: 'getaddrinfo',
   hostname: 'db',
   fatal: true
 }