is this a superset of features that integrates with docker compose?
Nope. Habitus fixes the things that are broken. Docker-compose is a way for composing your services (using images). Think of Habitus as the step before using Docker-compose. It helps create Docker images without leaving traces of secrets, libraries you don’t need, and other components utilized in the creation of a stable image before you start to compose your container services.
how does this compare to grammarly/rocker?
both projects fix stuff that's broken when it comes to creating docker images efficiently. With build.yml you can separate all the steps to make it more readable, and then put everything into the Dockerfile. Rocker has a similar sort of feature called the Rockfile. Habitus provides an additional solution for getting secrets in and out of your image and squashing the image afterwards.