docker build uri＃ref：dir是什么格式？

Git URL在其片段部分接受上下文配置，
用冒号分隔：。第一部分代表
Git将检出的引用，它可以是分支，标签或提交
SHA。第二部分表示存储库
中的一个子目录，该子目录将用作构建上下文。

Git URLs accept context configuration in their fragment section, separated by a colon :. The first part represents the reference that Git will check out, this can be either a branch, a tag, or a commit SHA. The second part represents a subdirectory inside the repository that will be used as a build context.

例如，运行此命令以使用一个名为docker的目录，
分支容器：

For example, run this command to use a directory called docker in the branch container:

docker build https://github.com/docker/rootfs.git#container:docker

https://docs.docker.com/engine/reference/commandline/build/