每个容器都有自己的网络接口和本地主机。因此，更改Java指向Postgres的方式：

Each container has its own network interface with its own localhost. So change how Java points to Postgres:

spring.datasource.url=jdbc:postgresql://localhost:5432/sample

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spring.datasource.url=jdbc:postgresql://db:5432/sample

db 将解析为正确的Postgres IP。

db will resolve to the proper Postgres IP.

奖金。使用docker-compose，您无需手动构建图片。因此更改：

Bonus. With docker-compose you don't need to build your image by hand. So change:

web:
  image: myuser/manager:latest

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web:
  build: .