I finally understand.
I have been avoiding Docker for a while now and sticking to VMs and LXC container templates.
Yesterday, I finally bit the bullet and was able to set up Docker with Portainer, and use Docker-Compose to create a stack with several containers, all connecting to the web through another that ran a VPN client.
I have quite a lot to learn, I spent several hours googling as I didn't understand that the existing dockercompose file from an image **does apply.** i.e. if you create a new image with a new dockerfile using **FROM** it will merge your new dockerfile with the one pulled from the image.
What an incredible thing docker is. Time to containerize ALL THE THINGS!