Commands to execute background process in Docker CMD

Besides the comments on your question that already pointed out a few things about Docker best practices you could anyway start a background process from within your start.sh script and keep that start.sh script itself in foreground using the nohup command and the ampersand (&). I did not try it with mongod but something like the following in your start.sh script could work:

#!/bin/sh...nohup sh -c mongod --dbpath /test &...

Of course there is also the official Docker documentation of how to start multiple services, again using a script file not the CMD. The docker documentation also states how to use supervisord as a process manager:

FROM ubuntu:latestRUN apt-get update && apt-get install -y supervisorRUN mkdir -p /var/log/supervisorCOPY supervisord.conf /etc/supervisor/conf.d/supervisord.confCOPY my_first_process my_first_processCOPY my_second_process my_second_processCMD ["/usr/bin/supervisord"]

If it is an option you could use a phusion base images which allows running multiple processes in one container. Thus you can run system services such as cron or other processes using a service supervisor like runit.

More information about whether or not a phusion base image is a good choice in your use case can be found here

A ruby focused description of how to avoid running more processes in your container except for your app you can find here. The elaborations are too detailed to repeat on SO.