So after hours of desperation I finally came to a solution (in case somebody has similar issues...):

1. I commented out all lines of code referring to crontab (#) in django settings.py module to avoid the error...
2. then i ran docker (to be able to exec bash):

docker compose up 3. so I could bring up the terminal of celery container $ docker compose exec celery bash 4. where I simply executed a pip install: $ pip install django-crontab 5. Finally - after a cheap Ctrl+C - I restarted the container (step.2)

And Voila, NO ERRORS!