try fixing the example using jessie as base image

doc/config_examples/bundle_with_extra_dependencies/.euts.cp.env

@@ -4,8 +4,9 @@
 COMPOSE_PROJECT_NAME=kezmbt_cp
 # We use Debian Jessie for the container OS as it comes with php 5.6 by default
 TESTSTACK_DEBIAN_VERSION=jessie
-# As of 2023/04, the default debian repos can not be used to update the docker image, as they are offline
-TESTSTACK_UPDATE_OS_ON_BUILD=false
+# As of 2023/04, the default debian repos for jessie can not be used to update the docker image, as they have been moved to 'archive'.
+# Otoh the freexian 'combined' mirror seems to work well
+TESTSTACK_DEBIAN_MIRROR=http://deb.freexian.com/extended-lts
 # We specify the php version too. This is redundant in this case and is only required for the teststack CI tests
 TESTSTACK_PHP_VERSION=5.6
 

docker/images/ez/Dockerfile

@@ -22,10 +22,11 @@ ARG timezone=none
 
 # Set up debian mirror
 # (use fixed debian mirrors if you have problems building on a given day)
-# @todo if we are on jessie or stretch, we might as well add the freexian elts repos. If we are on xenial, allow the
+# @todo if we are on jessie or stretch, we might as well just add the freexian elts repos. If we are on xenial, allow the
 #       users who have a pro subscription (free) to use those repos (see https://ubuntu.com/pro/beta)
 # -----------------------------------------------------------------------------
-RUN if [ "${debian_mirror}" != "none" ]; then printf "deb ${debian_mirror} ${debian_version} main\ndeb http://security.debian.org ${debian_version}/updates main\ndeb ${debian_mirror} ${debian_version}-updates main" > /etc/apt/sources.list; fi
+RUN if [ "${debian_mirror}" != "none" ]; then printf "deb ${debian_mirror} ${debian_version} main\n" > /etc/apt/sources.list; fi
+RUN if [ "${debian_mirror}" != "none" -a ${debian_version} != "jessie" -a ${debian_version} != "stretch" ]; then printf "deb http://security.debian.org ${debian_version}/updates main\ndeb ${debian_mirror} ${debian_version}-updates main\n" >> /etc/apt/sources.list; fi
 
 # Configure timezone
 # -----------------------------------------------------------------------------