Redirect logs from file to docker's output

Dockerfile

# To do that, you need to create symlink from your file (inside container) to docker's output file.

# Add bellow line to your Dockerfile
RUN ln -sf /proc/1/fd/1 /var/log/cronjob.log

# EXPLAIN:
# The first 1 is the PID of docker process.
# The second 1 is standard output


# Note: If you launch your container with --pid=host, please use following line
RUN ln -sf /proc/$$/fd/1 /var/log/cronjob.log

# EXPLAIN:
# $$ is the PID of the current script itself.

example.dockerfile

FROM php:7.2-fpm

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

# Create crontab file in the cron directory
RUN echo "* * * * * /usr/local/bin/php -r \"echo time() . PHP_EOL;\" >> /var/log/cronjob.log 2>&1" > /etc/cron.d/crontab

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

# Apply cron job
RUN crontab /etc/cron.d/crontab

# Create cronjob log file
RUN touch /var/log/cronjob.log

# Redirect cronjob log to docker log collector
RUN ln -sf /proc/$$/fd/1 /var/log/cronjob.log

CMD ["bash", "-c", "cron -f & php-fpm"]