How to run a cron job inside a docker container?

Question

I am trying to run a cronjob inside a docker container that invokes a shell script.

Yesterday I have been searching all over the web and stack overflow, but I could

Answer 1

Define the cronjob in a dedicated container which runs the command via docker exec to your service.

This is higher cohesion and the running script will have access to the environment variables you have defined for your service.

#docker-compose.yml
version: "3.3"
services:
    myservice:
      environment:
        MSG: i'm being cronjobbed, every minute!
      image: alpine
      container_name: myservice
      command: tail -f /dev/null

    cronjobber:
     image: docker:edge
     volumes:
      - /var/run/docker.sock:/var/run/docker.sock
     container_name: cronjobber
     command: >
          sh -c "
          echo '* * * * * docker exec myservice printenv | grep MSG' > /etc/crontabs/root
          && crond -f"