I would love to suggest to add the cronjob directly to the docker image, instead of relying on the host to run the job.

It could be as easy as adding the following to the Dockerfile:

COPY root /var/spool/cron/crontabs/root
CMD crond -l 2 -f

File root would simply contain our usual job:

*/5 * * * * php /usr/share/FreshRSS/app/actualize_script.php > /tmp/FreshRSS.log 2>&1

I'll happily work on a PR if this resonates well with you guys.