As a simple solution starting a Plone container with cron in foreground should work using
CMD [ "cron", "-f" ].
Also, you could modify the entrypoint
to support zodbpack, so it can be run direct.
Or modify the whole to run a configure and run a cron doing a zodbpack instead of the instance using a different command.
Latter would be something useful for the official image in some way (details to be discussed).