Add docs for reading Dockerfile from stdin.

Signed-off-by: Daniel Nephin <dnephin@docker.com>
Upstream-commit: 71b0f91129
Component: cli

components/cli/docs/reference/commandline/build.md

@@ -344,6 +344,13 @@ $ docker build -f Dockerfile.debug .
 This will use a file called `Dockerfile.debug` for the build instructions
 instead of `Dockerfile`.
 
+```bash
+$ curl example.com/remote/Dockerfile | docker build -f - .
+```
+
+The above command will use the current directory as the build context and read
+a Dockerfile from stdin.
+
 ```bash
 $ docker build -f dockerfiles/Dockerfile.debug -t myapp_debug .
 $ docker build -f dockerfiles/Dockerfile.prod  -t myapp_prod .