I cloned this repo on my Mac and it worked great. But when I tried it on my Windows 10 laptop running Docker and configured to use Linux containers, nothing ever showed up in /var/log/cron.log.

I'll continue to troubleshoot and update this issue with anything I learn.

I am doing exactly like in the example (but using debian:buster-slim instead of ubuntu:latest), but I run an additional command after cron

CMD ["cron"; "postgres", "-c", "hba_file=/etc/pg_hba.conf"]

The second command is started but not cron.

I disovered even if you have environement variable defined in the dockerfile or in the docker stack file , the environment variable will be available to shell if container but not to the cron. for that you would need to define env variable inside the cron file.

MYSQL_PASSWORD=secret_password
PYTHONPATH=/usr/local/app

*/45 * * * * python3 app.py >> /var/log/cron.log 2>&1
# An empty line is required at the end of this file for a valid cron file.

Thanks for posting this, it got me started. I found an approach (using crontab - < /path/to/crontab) that worked better for me. Thought I would post it here in case others found it useful.

modified   Dockerfile
@@ -1,19 +1,16 @@
 FROM ubuntu:latest
 MAINTAINER [email protected]
 
-# Add crontab file in the cron directory
-ADD crontab /etc/cron.d/hello-cron
-
-# Give execution rights on the cron job
-RUN chmod 0644 /etc/cron.d/hello-cron
-
 # Create the log file to be able to run tail
 RUN touch /var/log/cron.log
 
 #Install Cron
 RUN apt-get update
 RUN apt-get -y install cron
 
+# Install the crontab
+ADD crontab /tmp/crontab
+RUN crontab /tmp/crontab && rm /tmp/crontab
 
 # Run the command on container startup
 CMD cron && tail -f /var/log/cron.log
modified   crontab
@@ -1,2 +1,2 @@
-* * * * * root echo "Hello world" >> /var/log/cron.log 2>&1
+* * * * * echo "Hello world" >> /var/log/cron.log 2>&1
 # Don't remove the empty line at the end of this file. It is required to run the cron job