Connect the gate of an N-channel mosfet to pin GPIO 12, the souce to ground and connect the drain to the pin 4 of a PWM PC fan.
The default is really basic but should do a decent job.
However, you can change the control points by changing config.yml and rebuilding the image.
docker-compose build
docker-compose up
docker-compose -f docker-compose.yml -f docker-compose.dev.yml build
docker-compose -f docker-compose.yml -f docker-compose.dev.yml up
You'll run the code from the source directory and you'll get free verbose output.