解决Caused by: java.net.UnknownHostException: mysql: Name or service not knownorg.hibernate.exception

报错：

book-manager-app | 2026-04-26T10:06:31.294Z ERROR --- [main] o.h.engine.jdbc.spi.SqlExceptionHelper : Communications link failure book-manager-app | Caused by: java.net.UnknownHostException: mysql: Name or service not known book-manager-app exited with code 1

最后解决
启动顺序问题：虽然配置了depends_on，但 MySQL 可能还没完全就绪

# 1. 清理
docker compose down -v

# 2. 只启动 MySQL
docker compose up -d mysql

# 3. 等 MySQL 完全启动（15秒）
sleep 30

# 4. 再启动 app
docker compose up -d app

# 5. 查看日志
docker compose logs app

mysql 没准备好 等他准备好就行了