Yesterday, I was looking to add a cron to an existing Docker image, but it took longer than I would like. Even though I previously had a recipe for adding a cron to a Docker image, I had new contraints that made this solution unsuitable... 😐
Let's go over all the constraints I had:
- Using a Debian-based image
- The user running the image is a unprivileged user
- The cron is not the only process
- There is an entrypoint script
After searching on the internet, I finally found a working solution with the help of this Stack Overflow answer. So if anyone needs it, this article could serve as a reference. 😁
I hope again it will helps you as well! 😎
Dockerfile
Make sure to replace APPLICATION by the directory you want and NON_ROOT_USER by the name of your unprivileged user. 😉
# https://hub.docker.com/_/debian
FROM debian:buster-20210511
WORKDIR /opt/APPLICATION
# https://packages.debian.org/search?suite=buster&keywords=cron
# Install packages for our application
RUN apt-get update && apt-get -y install \
# Cron to periodically restart some services
cron=3.0pl1-134+deb10u1 \
&& echo "*/5 * * * * /opt/APPLICATION/restart.sh > /proc/1/fd/1 2>&1" >> /etc/cron.d/restart-cron \
# Give the necessary rights to the user to run the cron
&& crontab -u NON_ROOT_USER /etc/cron.d/restart-cron \
&& chmod u+s /usr/sbin/cron
COPY --chown=NON_ROOT_USER:NON_ROOT_USER . .
USER NON_ROOT_USER
ENTRYPOINT ["./entrypoint.sh"]
entrypoint.sh
#!/bin/bash
echo "Start cron"
cron
echo "cron started"
# Run forever
tail -f /dev/null
restart.sh
#!/bin/bash
echo "Restart the services..."
# ...
echo "Services restarted."
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