I had the issue of cronjobs not working (correctly) on my Docker instances.
This is what I did to fix it:
Set correct permissions: chmod 0600 /etc/cron.d/cronjob
Set correct owner: chown root /etc/cron.d/cronjob
When it failed, I could not find the logs of why it failed. In order to see the output of the failed cronjobs, I installed postfix (because the output of cronjobs is being mailed) and I installed rsyslog