diff --git a/container/Makefile b/container/Makefile
index c9f7c4a..97b33df 100644
--- a/container/Makefile
+++ b/container/Makefile
@@ -1,5 +1,5 @@
 ifeq ($(IMAGE_SRC),)
-IMAGE_SRC := docker://$(IMAGE_REPO)/$(IMAGE_NAME):$(IMAGE_TAG)
+IMAGE_SRC += docker://$(IMAGE_REPO)/$(IMAGE_NAME):$(IMAGE_TAG)
 endif
 
 $(IMAGE_NAME)-$(IMAGE_TAG):