You do realize that 'an entire init system' is a 100 line python script right? Packaging your application in a 'light-weight' way is not one of the big benefit

You do realize that 'an entire init system' is a 100 line python script right? Packaging your application in a 'light-weight' way is not one of the big benefits of Docker. Packaging your application in an isolated environment is _the_ big benefit of Docker, and that's exactly the benefit you get with this image. There's nothing monstrous about this image. It's based on the official Ubuntu image, has a few extra packages installed to satisfy some more dependencies so it's easier to use for other users, and Passenger itself is just a few MB. You probably don't get Docker very well. The idea is that you start a Docker container, and after a while a service is online on the port you EXPOSE-d. If a container satisfies those properties, then it's as composable and reproducible as any other. It's the magic of Docker.