diff --git a/Makefile b/Makefile
index 00ffc6b..48611eb 100644
--- a/Makefile
+++ b/Makefile
@@ -9,8 +9,8 @@ CONTAINER_RUNTIME?=$(shell which podman)
 
 # MARKDOWNLINT_IMAGE_REGISTRY_NAME
 # Defines the name of the new container to be built using several variables.
-MARKDOWNLINT_IMAGE_REGISTRY_NAME:=git.cryptic.systems
-MARKDOWNLINT_IMAGE_REGISTRY_USER:=volker.raschek
+MARKDOWNLINT_IMAGE_REGISTRY_NAME?=git.cryptic.systems
+MARKDOWNLINT_IMAGE_REGISTRY_USER?=volker.raschek
 
 MARKDOWNLINT_IMAGE_NAMESPACE?=${MARKDOWNLINT_IMAGE_REGISTRY_USER}
 MARKDOWNLINT_IMAGE_NAME:=markdownlint