Comment on "ubuntu" user

Author: amarburg

.docker/Dockerfile

@@ -39,7 +39,9 @@ RUN apt-get -q update \
 
 FROM ci AS robot
 
-# Configure a new non-root user
+#
+# Ubuntu 24.04 "Noble", which is used as the base image for
+# jazzy and rolling images, now includes a user "ubuntu" at UID 1000
 ARG USERNAME=ubuntu
 ARG USER_UID=1000
 ARG USER_GID=$USER_UID