How to Build a Docker Image For Running a Cron Job

In automation world there are many ways to automate a task and in most cases the automation need scheduler function. You might need to automate a simple scheduled task such as sending a periodic email or something more complicated such as buying or selling cryptocurrencies based on 200 days of bitcoin moving average.

Cron job is the simplest and easiest way to schedule a task. In certain cases you might want to have a container that runs a cron service to execute some tasks.

This article is exactly about that 😁. Let’s dive deeper.

Creating Dockerfile

In this example I use a python image as base. You need to replace few things from following Dockerfile code.

• You need to replace cool_task.cron with your cron file

FROM python:3.7.9-buster
USER root
RUN apt-get update -y \
  && apt-get install cron -y
COPY cool_task.cron /etc/cron.d/cool_task.cron
RUN chmod 0644 /etc/cron.d/cool_task.cron \
  && crontab /etc/cron.d/cool_task.cron
CMD ["cron", "-f"]

An example of cron file that executes a shell script file buy_bitcoin.sh every 2 minutes

*/2 * * * * bash /buy_bitcoin.sh >> ~/cron.log 2>&1

Building the image

sudo docker build . -t your_repo/py-cron:latest

Creating docker-compose.yml

You need to replace several things from this example:

• You need to specify your volume mount (if required)

# docker-compose.yml
version: '3'
services:
  py-cron:
    image: your_repo/py-cron:latest
    container_name: py-cron

Running docker-compose

sudo docker-compose up -d

Enjoy your new scheduler in a Docker container.