[libc++] Fix the Docker image build

As a drive-by, also fix a comment that was outdated.

Author: ldionne

libcxx/utils/ci/Dockerfile

@@ -46,9 +46,8 @@ FROM $BASE_IMAGE AS builder-base
 
 # Changing this file causes a rebuild of the image in a GitHub action.
 # However, it does not cause the CI runners to switch to that image
-# automatically, that must be done by updating the SHA in the Github workflow
-# file. The date uses the ISO format YYYY-MM-DD.
-RUN echo "Last forced update executed on 2025-04-28."
+# automatically, to do that please refer to the documentation.
+RUN echo "Last forced update executed on 2025-07-25."
 
 # Make sure apt-get doesn't try to prompt for stuff like our time zone, etc.
 ENV DEBIAN_FRONTEND=noninteractive