diff --git a/factory/factory.Dockerfile b/factory/factory.Dockerfile
index 6ebe126819..e01f8caad3 100644
--- a/factory/factory.Dockerfile
+++ b/factory/factory.Dockerfile
@@ -1,5 +1,7 @@
-# Set base image arg to allow easy testing of other debian versions.
-ARG BASE_IMAGE
+# BASE_IMAGE is expected to be overridden
+# Regular builds, using docker compose, take the value from
+# the .env file in the same directory as this file
+ARG BASE_IMAGE='debian:12-slim'
 
 FROM ${BASE_IMAGE} AS factory