Generate an error on unknown Dockerfile instruction

Instead of just printing a warning and going on, this will generate
an error and stop processing.

This used to be part of #10561 but I decided it might need its own
independent discussion/PR as to not derail #10561.

Signed-off-by: Doug Davis <dug@us.ibm.com>

docs/sources/release-notes.md

@@ -15,6 +15,9 @@ For a complete list of patches, fixes, and other improvements, see the
 
 *New Features*
 
+* [1.6] The Docker daemon will no longer ignore unknown commands
+  while processing a `Dockerfile`. Instead it will generate an error and halt
+  processing.
 * The Docker daemon has now supports for IPv6 networking between containers
   and on the `docker0` bridge. For more information see the
   [IPv6 networking reference](/articles/networking/#ipv6).
@@ -22,7 +25,7 @@ For a complete list of patches, fixes, and other improvements, see the
   container to writing to volumes [PR# 10093](https://github.com/docker/docker/pull/10093).
 * A new `docker stats CONTAINERID` command has been added to allow users to view a
   continuously updating stream of container resource usage statistics. See the
-  [`stats` command line reference](/reference/commandline/cli/#stats) and the 
+  [`stats` command line reference](/reference/commandline/cli/#stats) and the
   [container `stats` API reference](/reference/api/docker_remote_api_v1.17/#get-container-stats-based-on-resource-usage).
   **Note**: this feature is only enabled for the `libcontainer` exec-driver at this point.
 * Users can now specify the file to use as the `Dockerfile` by running