I’m excited that they announced this yesterday: https://blog.heroku.com/archives/2015/5/5/introducing_heroku_docker_release_build_deploy_heroku_apps_with_docker
The major complaint most people have about Heroku is that as your app gets bigger it isn’t that cheap to run on Heroku and it can be hard to move somewhere else. In my mind this Docker support allows you to start your app in the free or low level Heroku tiers and if you outgrow it - easily move on to your own server (or another Docker-supporting platform).
You can check out the source at https://github.com/heroku/heroku-docker - in particular the currently supported languages (Node, Ruby, etc) are listed under heroku-docker/tree/master/platforms