Dockerfile for cron on Debian

FROM php:7.2-fpm-buster

RUN apt-get update && \
    apt-get install -y cron

# TLDR: CRON and Docker don't play well together. 
# 
# The idiomatic way to handle logging in docker is to output to stdout. This is achieved by
# redirecting the program output to /proc/1/fd/1 e.g. * * * * * echo hello > /proc/1/fd/1 2>&1. However, if you have a 
# non-root user cron job, this is not possible due to the non-root user not being able to write to /proc/1/fd/1 which 
# is owned by the root user (cron must be executed as root or it won't have permission to start jobs on behalf of other 
# users). The best solution to this problem is to redirect the output of the cron job to a log file, and 
# have the docker command perform a tail of the logfile. This comes with its own issues though, namely, the 
# log file will grow and grow until the container restarts, or explodes. To avoid this problem we truncate the log 
# periodically.

RUN touch /var/log/cron.log && \
    chown www-data /var/log/cron.log && \
    chmod 644 /var/log/cron.log

USER www-data
RUN echo "* * * * * > /var/log/cron.log" | crontab -
RUN echo "* * * * * /usr/local/bin/php /var/www/artisan schedule:run >> /var/log/cron.log" | crontab -

USER root

CMD cron && tail -f /var/log/cron.log