I am trying various open source PaaS based on Docker and so far the easiest one is Dokku.

The problem with Dokku is that it does not manage scaling of your app by launching more containers with yours application. You can install third-party plugins, which will start multiple processes inside single docker container, but this doesn't work well for web apps listening on one port. And I could not find how to leverage SO_REUSEPORT in Express.js app.