Merge pull request #4187 from creack/fix_hostflag_panic

Fix DOCKER_HOST=tcp:// panic

docker/docker.go

@@ -58,6 +58,9 @@ func main() {
 			// If we do not have a host, default to unix socket
 			defaultHost = fmt.Sprintf("unix://%s", api.DEFAULTUNIXSOCKET)
 		}
+		if _, err := api.ValidateHost(defaultHost); err != nil {
+			log.Fatal(err)
+		}
 		flHosts.Set(defaultHost)
 	}