heroku.yml now supports targeting a stage from a multi-stage Docker build
Change effective on 27 November 2018
With a multi-stage Docker build you can manage build and production images for your application in a single
Dockerfile. For example, you might want to have specific packages or database migration scripts only available at build and release time, but not in your final production image.
FROM heroku/heroku:18-build AS builder ... FROM heroku/heroku:18 AS production ...