Inspired by this post, from the code here

I wanted to be able to use opencv inside a docker container

Running:

Have docker installed now make build

This was only tested on Kubuntu, but should work on other OS's sudo xhost +

and run it with a make run

2 windows will appear, tweak the sliders until you only see your 'pen'

Hit the s key, this will safe the numpy array, the next window will read from this file

If you want to debug, just do a make shell