So I have this exe file that I’d like make into a docker image so that I can run it in portainer on my TrueNAS Scale system.
Does anyone know of a good guide for doing this?
Never mind, solved the issue another way. 
It is great that you solved the issue. I didn’t have time to read it until now, but I couldn’t have answared as your goal was not clear. An exe can mean man things. If it is a windows app, whether it works in a container depends on the app. copying the exe would be easy, running it not always. But since you menitoned TrueNAS, I assume you wanted to run Linux containers, so a Windows app would not run on it.