Replace ADD with COPY in Dockerfile

Docker-DCO-1.1-Signed-off-by: Tibor Vass <teabee89@gmail.com> (github: tiborvass)

Dockerfile

@@ -101,4 +101,4 @@ ENV	DOCKER_BUILDTAGS	apparmor selinux
 ENTRYPOINT	["hack/dind"]
 
 # Upload docker source
-ADD	.	/go/src/github.com/dotcloud/docker
+COPY	.	/go/src/github.com/dotcloud/docker