Dockerfile: buildtags for old btrfs

Since the build uses ubuntu 14.04, which has an old btrfs, include the
buildtags needed for this old version to not break the build.

Signed-off-by: Vincent Batts <vbatts@redhat.com>

Dockerfile

@@ -103,7 +103,7 @@ RUN useradd --create-home --gid docker unprivilegeduser
 
 VOLUME	/var/lib/docker
 WORKDIR	/go/src/github.com/docker/docker
-ENV	DOCKER_BUILDTAGS	apparmor selinux
+ENV	DOCKER_BUILDTAGS	apparmor selinux btrfs_noversion
 
 # Wrap all commands in the "docker-in-docker" script to allow nested containers
 ENTRYPOINT	["hack/dind"]