Full credit goes to prakhar1989, I've only followed his tutorial here: https://prakhar.me/docker-curriculum/#our-image
After running with:
docker run -d -p 8888:5000 tengu8890/cats
it displays random cat gif. Of course you can change ports to suit your needs - 8888 is external, 5000 is internal.
You can also run it with --restart=always to keep it online automatically after reboot.