Cron comes with just a metric ton of recommended dependencies including
mariadb-common which is just a bunch of unneccessary weight. Just
install what's necessary for cron.
Adds a pruning script which is installed but not set to run by default.
Also adds for that script that can be run in a container that replicates the db container's conditions
init.d isn't run when docker containers are run so we need to modify the entrypoint.
This commit makes the ugly choice of injecting the command in automatically in order to avoid the need to manually maintain the images entrypoint.
