diff --git a/docker/base-dev-x.dockerfile b/docker/base-dev-x.dockerfile
index 5c0a3192e..87c7aa9db 100644
--- a/docker/base-dev-x.dockerfile
+++ b/docker/base-dev-x.dockerfile
@@ -1,3 +1,5 @@
+# Default BASE_IMAGE allows standalone `docker build`.
+# build.sh overrides this via --build-arg for each target.
 ARG BASE_IMAGE=ghcr.io/microsoft/ark/ark:base-cuda12.1
 FROM ${BASE_IMAGE}
 
diff --git a/docker/base-x.dockerfile b/docker/base-x.dockerfile
index c5ff03afe..52a7d267e 100644
--- a/docker/base-x.dockerfile
+++ b/docker/base-x.dockerfile
@@ -1,3 +1,5 @@
+# Default BASE_IMAGE allows standalone `docker build`.
+# build.sh overrides this via --build-arg for each target.
 ARG BASE_IMAGE=nvidia/cuda:12.1.1-devel-ubuntu20.04
 FROM ${BASE_IMAGE}