Updating the default https://github.com/docker-library/rails onbuild container to support entrypoints.
By simply adding an entrypoint script, and moving the bundle install to entrypoint, along with the ability to run db migrate db setup, the container is able to be run with the usr/src/app mounted as a volume. This allows the container to be rerun with the latest changes to your app included.
i.e. docker run --rm -v "$PWD":/usr/src/app -w /usr/src/app your_onbuild_image will just work™