Revert "Engine reference BUILD tarball contexts mangled"

engine/reference/commandline/build.md

@@ -96,9 +96,11 @@ Build Syntax Suffix             | Commit Used           | Build Context Used
 
 If you pass an URL to a remote tarball, the URL itself is sent to the daemon:
 
+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`:
+
 ```bash
 $ docker build http://server/context.tar.gz
-```
 
 The download operation will be performed on the host the Docker daemon is
 running on, which is not necessarily the same host from which the build command