If you put the cron jobs you want in a file with your sources you can do:
COPY /host/path/to/cron.conf /some/path/in/docker/container/cron.conf
Then you can do
RUN crontab /some/path/in/docker/container/cron.conf
That will import your cron config to your crontab.
You can double check this by running contab -e inside your container.
I know it's a "bit" late, but hope it helps 🙂