diff --git a/build.sh b/build.sh
index 5ab4cfd1..071a82a8 100755
--- a/build.sh
+++ b/build.sh
@@ -431,7 +431,9 @@ updates() {
 probe() {
     # Probe the build environment and set up variables
     if [ ! -e /etc/openslide-winbuild-builder-v1 ] && [ ! -e /etc/openslide-winbuild-builder-v2 ]; then
-        echo "Must run inside the builder container.  See README.md."
+        echo "Not running in a compatible builder container.  Either build.sh isn't running"
+        echo "in the container (see instructions in README.md) or the container image is too"
+        echo "old or too new."
         exit 1
     fi
 
diff --git a/builder/linux/Dockerfile b/builder/linux/Dockerfile
index 94bb2d67..d8f2c2b0 100644
--- a/builder/linux/Dockerfile
+++ b/builder/linux/Dockerfile
@@ -1,5 +1,7 @@
 # Use a Linux distro with an older glibc for maximum compatibility
 FROM quay.io/almalinuxorg/almalinux:8
+# NOTE: try to keep the current container image compatible with the latest
+# stable source release, so people can conveniently build from the source zip
 RUN touch /etc/openslide-linux-builder-v1
 RUN dnf -y upgrade && \
     dnf -y install 'dnf-command(config-manager)' epel-release && \
diff --git a/builder/windows/Dockerfile b/builder/windows/Dockerfile
index 348287f0..4ff490dc 100644
--- a/builder/windows/Dockerfile
+++ b/builder/windows/Dockerfile
@@ -1,4 +1,6 @@
 FROM docker.io/gentoo/stage3:latest
+# NOTE: try to keep the current container image compatible with the latest
+# stable source release, so people can conveniently build from the source zip
 RUN touch /etc/openslide-winbuild-builder-v2
 RUN echo 'FEATURES="-sandbox -usersandbox -ipc-sandbox -network-sandbox -pid-sandbox"' >> /etc/portage/make.conf
 COPY package.accept_keywords /etc/portage/package.accept_keywords/cross