OK I think we're getting somewhere.
If you stop and start the container, is a new backup taken? (It will be 5-10 minutes after restart)
If so, please edit /etc/cron.d/anacron so it reads
# /etc/cron.d/anacron: crontab entries for the anacron package
30 7 * * * root anacron || :
I think what's happening is cron is running, but anacron (which we're using to ensure backups are taken even if the containers aren't left running 24/7) is being run once, and then when cron tries to run it, it's being invoked wrongly (via start -q).
If so, I can add a step to patch that in the build and release an updated container.