return 404 on no such containers in /attach

Upstream-commit: e5fa4a4956393afc473539710e2a4b7297eb6ebf
Component: engine

components/engine/api.go

@@ -541,6 +541,10 @@ func postContainersAttach(srv *Server, version float64, w http.ResponseWriter, r
 	}
 	name := vars["name"]
 
+	if _, err := srv.ContainerInspect(name); err != nil {
+		return err
+	}
+
 	in, out, err := hijackServer(w)
 	if err != nil {
 		return err