docker build url#ref:dir
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.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/