This Mongo image is based around wanting Server Density to run on the container. Server Density requires Python to run, so this automatically installs Python as part of the process. Cron is also installed, so that a crontab file can be used to keep the Server Density agent running.
As part of the Server Density agent install, the checks.py and config.cfg files are moved to /etc/sd-agent/replaced-files/ so that custom ones can be used instead. You'll want to create the links as below, then copy these files and edit them as appropriate. You may not need to make any changes to checks.py, but config.cfg will need the agent key and Server Density URL adding.
Suggested volume links (image/container path):
/etc/sd-agent/checks.py # this is the file that tells Server Density what checks to do
/etc/sd-agent/config.cfg # this is the file that tells Server Density its configuration
/data/db # for Mongo