crontab

0 1 * * * root . /root/envs.sh;/bin/backup.sh >> /var/log/cron.log 2>&1

Dockerfile

FROM ubuntu:latest

ADD start.sh /bin/start.sh
RUN chmod +x /bin/start.sh

# Add crontab file in the cron directory
ADD crontab /etc/cron.d/thecron

# Give execution rights on the cron job
RUN chmod 0644 /etc/cron.d/thecron

# Create the log file to be able to run tail
RUN touch /var/log/cron.log

# start script
CMD /bin/bash /bin/start.sh

start.sh

# export all environment variables to use in cron
env | sed 's/^\(.*\)$/export \1/g' > /root/envs.sh
chmod +x /root/envs.sh

# Run the command on container startup
cron && tail -f /var/log/cron.log