By default Trellis only runs composer install on deployments. We can’t really make any assumptions about a theme’s asset workflow so we don’t.
https://github.com/roots/roots-example-project.com contains the necessary configuration to get Trellis working with Sage as an example. We suggest running those commands locally and then copying the files.
Currently that repo is outdated 
but see here for working new code: Deployment with the new Trellis hooks