add github.com/docker/docker dependency

vendor/github.com/docker/distribution/doc.go

@@ -0,0 +1,7 @@
+// Package distribution will define the interfaces for the components of
+// docker distribution. The goal is to allow users to reliably package, ship
+// and store content related to docker images.
+//
+// This is currently a work in progress. More details are available in the
+// README.md.
+package distribution