Run Cron jobs inside Docker container

2022-02-03

This Dockerfile can be used to execute python code, scheduled by a Cron job.

The basic steps followed, load a random python application which is executed from the index.py file.
Make sure cron is installed in the image, load environment variables into a env file, specifically created to load the environment variables (restrict unix permissions) needed by the python app, availabe to os.environ().
The python app start command is written into an executable file script.sh, whose execution will be finally scheduled by cron.

	FROM python:3.8.12-slim-buster

	RUN apt update && apt install -y cron vim
	RUN mkdir /code

	ARG DB_HOST
	ARG DB_NAME
	ARG DB_USER
	ARG DB_PASS
	ARG CRON_SCHEDULE
	ENV DB_HOST=${DB_HOST}
	ENV DB_NAME=${DB_NAME}
	ENV DB_USER=${DB_USER}
	ENV DB_PASS=${DB_PASS}
	ENV CRON_SCHEDULE=${CRON_SCHEDULE:-"* * * * *"}

	COPY ./app /code/app
	COPY index.py /code/

	WORKDIR /code
	RUN touch .myenv
	RUN echo "export DB_HOST=${DB_HOST_REMOTE}" >> .myenv
	RUN echo "export DB_PASS=\"${DB_PASS_REMOTE}\"" >> .myenv
	RUN echo "export DB_USER=${DB_USER_REMOTE}" >> .myenv
	RUN echo "export DB_NAME=${DB_NAME_REMOTE}" >> .myenv
	RUN echo "source /code/.myenv" > load.sh && chmod +x load.sh

	RUN echo "python /code/index.py >> /code/cron.log 2>&1 &" > script.sh

	RUN chmod +x script.sh
	RUN touch cron.log && chmod 664 cron.log
	RUN crontab -l \| { cat; echo "PATH=/bin/:/usr/bin:/usr/local/bin:$PATH\n\n\
	${CRON_SCHEDULE} BASH_ENV=/code/load.sh bash /code/script.sh\n\
	"; } \| crontab -

	CMD /etc/init.d/cron start && echo "Listening log..." && tail -f /code/cron.log

view raw Dockerfile.cron hosted with ❤ by GitHub

The BASH_ENV environment variable can be used to preload environment variables at the cronjob context.

Environment varaibles loaded with BASH_ENV are only available to the cron job loading the BASH_ENV variable. In fact, entries in the crontab won’t share the same cron job context.

Code

Run Cron jobs inside Docker container