For those who wants to use a simple and lightweight image:
FROM alpine:3.6
# copy crontabs for root user
COPY config/cronjobs /etc/crontabs/root
# start crond with log level 8 in foreground, output to stderr
CMD ["crond", "-f", "-d", "8"]
Where cronjobs is the file that contains your cronjobs, in this form:
* * * * * echo "hello stackoverflow" >> /test_file 2>&1
# remember to end this file with an empty new line