How to schedule a job in the background and start another process inside a docker container?

My question is quite close to other questions such as this one and this one which use cron or an infinite loop to schedule single a job/process inside a docker container.

Both approaches work for me but my problem is a bit different: I would like to

schedule a job/process in the background
and subsequently start another process.

In my real world problem:

is an ETL process and
is a Django instance (websever).

How can I do this in a clean way?

Any hints are welcome!

Solution

I found a solution using docker-compose based on the following article.

It basically overrides the entrypoint in another service as follows in the docker-compose.yml file:

version: "3"

services:
  app:
    image: demo-image:latest
    volumes:
      - data:/app-data
  cron:
    image: demo-image:latest
    command: [ "cron -f" ]
    tty: true
    volumes:
      - data:/app-data

volumes:
  data:

My example Dockerfile:

# syntax=docker/dockerfile:experimental
FROM python:3.9

RUN apt-get update
RUN apt-get -y install cron
COPY my-crontab /etc/cron.d/my-crontab
RUN chmod 0744 /etc/cron.d/my-crontab
RUN crontab -l | { cat; cat /etc/cron.d/my-crontab } | crontab -
RUN touch /var/log/cron.log
WORKDIR /code
COPY . /code
ENTRYPOINT ["/bin/bash", "/docker-entrypoint.sh"]

My example cronjob file with an important hint that took me hours of bugtracking:

* * * * * echo "Hello world" >> /var/log/cron.log 2>&1
# must be ended with a new line "LF" (Unix) and not "CRLF" (Windows)

I found this solution much cleaner because it only uses one process per container.