Unfortunately Trellis uses the
git archive command to populate a new release directory and it does not support submodules.
You’d probably have to define a custom hook in
deploy_prepare_after which did some extra manual work to copy over submodules. I assume you might need to delete/clear the release directory Trellis makes then do a normal recursive git clone.
See https://roots.io/trellis/docs/deploys/#custom-tasks for more details on customizing hooks.