I am the author of passenger-docker. It is not true that this image is like creating a virtual machine, or that it is not lightweight. The image is very-much in line with the Docker philosophy, as I’ve described in this blog post: http://blog.phusion.nl/2015/01/20/baseimage-docker-fat-conta...

You talked about “an entire init system”, but this init system is a lightweight, stripped down variant written especially for Docker. The entire thing is just a few hundred lines Python script. You may be thinking that an init system is not necessary in Docker, but that is not true, as I’ve explained in “Docker and the PID 1 zombie reaping problem”: http://blog.phusion.nl/2015/01/20/docker-and-the-pid-1-zombi...

With passenger-docker, you only create an image with your web app in it, plus an app server to run that web app. Nothing more. The database is still external. Memcached and Redis are not installed unless you explicitly enable them. The image is lightweight as you can expect from Ruby, Python and Node.js apps.

In fact, we've explicitly split the image 8 variants, each variant containing only the minimum possible: https://github.com/phusion/passenger-docker#image_variants

Passenger-docker is very much in line with the idea of composable, reproducible infrastructure. Nothing in passenger-docker violates those principles.