Use Alpine's BusyBox cron instead of go-cron

[eeshugerman]

It makes me a little uneasy that the cron we're using is just a binary curled from a hardcoded GitHub release URL of a little-known project.

Instead, we should do something like this: https://stackoverflow.com/a/47960145

This would probably be a breaking change (go-cron offers some special-sauce scheduling syntax, IIRC), so it would be a good opportunity to switch to a new image tagging convention which incorporates a version for postgres-backup-s3 itself, in addition to the PostgreSQL version.