I don't think there's a writeup out there, but mrsk just uses docker under the hood. So, if you have a CMD in your Dockerfile, it will use that.
If you have an image that can run multiple things, like a rails app that can run the app process for web traffic by default, but it can also run job workers with the right command, you can provide the cmd in the mrsk config. You can see this in the jobs role in the example: https://github.com/mrsked/mrsk#using-different-roles-for-ser....