Cannot make/remove an entry for the specified session - cron
That error is usually due to a pam security issue.
It has been fixed in debian recently: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=726661 and in Ubuntu Wily (15.10).
As a workaround you can try commenting the module pam_loginuid.so inside /etc/pam.d/cron and restart cron (or the docker container).
You can use something similar to this in your Dockerfile:
RUN sed -i '/session required pam_loginuid.so/c\#session required pam_loginuid.so' /etc/pam.d/cron