Docker: cron within container no errors but still not working - linux

Question

I am new to docker. Managed to create a script which gets info from online source and populates an SQL DB. All works well within docker container.
However, I need to make this to run for example every minute.
So I amended my working Dockerfile and added the following:
Dockerfile:
RUN apt-get install -y cron
COPY cronms /etc/cron.d/cronms
RUN chmod 0644 /etc/cron.d/cronms
RUN crontab /etc/cron.d/cronms
RUN touch /var/log/cron.log
CMD cron && tail -f /var/log/cron.log
#CMD [ "python3", "./my_script.py" ] -- command before cron
my cronms file:
*/1 * * * * /usr/bin/python3 /my_script.py
Image builds without errors however when running it data is not being downloaded.
What I am missing please?
Thanks

Answer 1

Your crontab file has incorrect syntax. From the cron man page:
The system crontab (/etc/crontab) and the packages crontabs (/etc/cron.d/*) use the same format, except that the username for the command is specified after the time and date fields and before the command.
So your cronms file should look like:
*/1 * * * * root /usr/bin/python3 /my_script.py
I assume the Dockerfile in your question is truncated, but it's worth noting that for this to work you may need to explicitly install python3 as well as cron, depending on which base image you're using.
If I build an image using this Dockerfile:
FROM ubuntu:20.04
RUN apt-get update
RUN apt-get install -y cron python3
COPY cronms /etc/cron.d/cronms
RUN chmod 0644 /etc/cron.d/cronms
RUN crontab /etc/cron.d/cronms
RUN touch /var/log/cron.log
COPY my_script.py /my_script.py
CMD cron && tail -f /var/log/cron.log
And this cronms:
*/1 * * * * root /usr/bin/python3 /my_script.py
And this my_script.py:
#!/usr/bin/python3
import time
with open("/tmp/datafile", "a") as fd:
fd.write(time.ctime())
fd.write("\n")
Then I can confirm that everything works as expected: the script is
executed once a minute.
Note that nothing is written to /var/log/cron.log, however,
because cron logs to syslog, not to a file. To see output from
cron, you would need to arrange to run a syslog daemon, or use a
different cron daemon (for example, the busybox crond command can
log to a file or stderr).