fix in makefile

Change-Id: Iac1f9b70eccb784823dc2f75a858bf0bd2849464
diff --git a/Makefile b/Makefile
index 6a02798..4abf9b1 100644
--- a/Makefile
+++ b/Makefile
@@ -103,7 +103,7 @@
 	docker build -t cord/onos -f docker/Dockerfile.onos docker
 
 tester:
-	docker build -t cord/onos -f docker/Dockerfile.tester docker
+	docker build -t cord/tester -f docker/Dockerfile.tester docker
 
 protos:
 	make -C voltha/protos