Use git url fragment to specify reference and dir context.
@@ -659,6 +659,29 @@ After the command succeeds, the directory is sent to the Docker daemon as the co
Local clones give you the ability to access private repositories using
local user credentials, VPN's, and so forth.
+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
+
+The following table represents all the valid suffixes with their build contexts:
+
+Build Syntax Suffix | Commit Used | Build Context Used
+--------------------|-------------|-------------------
+`myrepo.git` | `refs/heads/master` | `/`
+`myrepo.git#mytag` | `refs/tags/mytag` | `/`
+`myrepo.git#mybranch` | `refs/heads/mybranch` | `/`
+`myrepo.git#abcdef` | `sha1 = abcdef` | `/`
+`myrepo.git#:myfolder` | `refs/heads/master` | `/myfolder`
+`myrepo.git#master:myfolder` | `refs/heads/master` | `/myfolder`
+`myrepo.git#mytag:myfolder` | `refs/tags/mytag` | `/myfolder`
+`myrepo.git#mybranch:myfolder` | `refs/heads/mybranch` | `/myfolder`
+`myrepo.git#abcdef:myfolder` | `sha1 = abcdef` | `/myfolder`
+
Instead of specifying a context, you can pass a single Dockerfile in the
`URL` or pipe the file in via `STDIN`. To pipe a Dockerfile from `STDIN`:
