See City-of-Helsinki/drupal-docker-images to see how this image is built.
Crons are run inside a separate container and use docker/openshift/crons/base.sh as an entrypoint.
Base.sh will be run only once and the container will die as soon as the "main" loop is finished, meaning that all scripts must be run inside an infinite loop. You can use sleep XX to define how often certain task should be run.
Any scripts placed in repository's docker/openshift/crons folder will be copied automatically
to /crons folder inside the cron container, but won't be run automatically. See docker/openshift/crons/base.sh for more information about how to execute custom cron scripts.