参考教程：https://docs.docker.com/develop/develop-images/dockerfile_best-practices/
环境
- virtual box 6.1
- centos 7.8
- docker 19.03
ONBUILD
An ONBUILD command executes after the current Dockerfile build completes. ONBUILD executes in any child image derived FROM the current image. Think of the ONBUILD command as an instruction the parent Dockerfile gives to the child Dockerfile.
当前的 Dockerfile 构建完成后，将执行 ONBUILD 命令。ONBUILD 在派生自 FROM 的任何子镜像中执行。可以将 ONBUILD 命令视为父 Dockerfile 给子 Dockerfile 的指令。
A Docker build executes ONBUILD commands before any command in a child Dockerfile.
Docker 构建在子 Dockerfile 中的任何命令之前执行 ONBUILD 命令。
ONBUILD is useful for images that are going to be built FROM a given image. For example, you would use ONBUILD for a language stack image that builds arbitrary user software written in that language within the Dockerfile, as you can see in Ruby’s ONBUILD variants.
ONBUILD 对于将要构建的镜像 FROM 给定镜像很有用。例如，您可以将 ONBUILD 用于语言堆栈映像，以构建在 Dockerfile 中以该语言编写的任意用户软件，如 Ruby’s ONBUILD variants。
Images built with ONBUILD should get a separate tag, for example: ruby:1.9-onbuild or ruby:2.0-onbuild.
用 ONBUILD 构建的图像应获得一个单独的标签，例如：ruby:1.9-onbuild 或 ruby:2.0-onbuild。
Be careful when putting ADD or COPY in ONBUILD. The “onbuild” image fails catastrophically if the new build’s context is missing the resource being added. Adding a separate tag, as recommended above, helps mitigate this by allowing the Dockerfile author to make a choice.
将 ADD 或 COPY 放入 ONBUILD 时要小心。如果新构建的上下文缺少要添加的资源，则 “onbuild” 映像将灾难性地失败。如上所述，添加一个单独的标签可以通过允许 Dockerfile 作者做出选择来缓解这种情况。
总结
介绍了 Dockerfile 的 ONBUILD 指令的最佳实践。