How to build a cron docker image properly?

The only difference between using COPY and RUN are the permissions on the /etc/crontab file: with COPY this is 664 and with RUN 644.

I cannot find anything on permissions that /etc/crontab needs to have but if you add

RUN chmod 644 /etc/crontab

after the COPY line in your Dockerfile the cronjobs run (at least for me). So I think the permissions have to be 644