Crontab doesn't meet expected timing

[mike2307]

Since the upgrade from Nextcloud 16 to 17, it seems that the expected cron interval has changed from 15 to 5 minutes.
The "Basic Settings" page in the admin area states:

Use system cron service to call the cron.php file every 5 minutes. The cron.php needs to be executed by the system user "www-data".

However, the crontab inside the Docker image uses the previously expected interval of 15 minutes:
https://github.com/nextcloud/docker/blob/master/17.0/apache/Dockerfile#L16

This causes a red warning message in the "Background jobs" section on the "Basic Settings" page in the admin area:

Last job execution ran 10 minutes ago. Something seems wrong.

If the new cron interval requirement is 5 minutes, the Dockerfile shall be updated accordingly.