I would like to build a custom image for the Joule. It doesn't look like the ostro project has Joule support in master yet. Is there a fork published somewhere with Joule specific bits?
We're working to improve the documentation and resources available for Joule. As you said, Ostro project doesn't have Joule support in master yet. The information available about Joule can be found here
More information about Joule will be posted soon.