Unfortunately I get an error about a missing cargo.toml :( Is it possible you need to copy one to the Docker image to get this to work?

(I tested with rake test spwn after cloning your branch)

Whoops, now that I think about it, git clone clones the repository into a new folder instead of the current one, therefore it's quite obvious that it's not finding it 😂
I'll update this PR when I'm back from vacation.