It’s the backend that implements serverless architecture. A serverless server, I guess. Roll your eyes if you like, but “serverless” is still a snappier term than “declarative on-demand server provisioning, configuration, and scaling” and most people are into that whole brevity thing.
Except Dokku doesn't do those things. Dokku doesn't scale your app automatically and it doesn't shut it off when it's not being used. It runs your web server process continuously, handles some 12 factor config, and does some nginx stuff for you. Until this year it didn't support managing a cluster at all and was entirely focused on single box deploys. The scale command just runs more processes on the same machine which if you're not using Node is probably not even a good idea to do.
