In line with Phusion Passenger's goal, passenger-docker's goal is to make Docker image building for web apps much easier and faster.
Why is this image called "passenger"? It's to represent the ease: you just have to sit back and watch most of the heavy lifting being done for you. Passenger-docker is part of a larger and more ambitious project: to make web app deployment ridiculously simple, to heights never achieved before.
It reduces the time needed to write a correct Dockerfile. You won't have to worry about the base system and the stack, you can focus on just your app.
It sets up the base system correctly. It's very easy to get the base system wrong, but this image does everything correctly. Learn more.
It drastically reduces the time needed to run docker build, allowing you to iterate your Dockerfile more quickly.
It reduces download time during redeploys. Docker only needs to download the base image once: during the first deploy. On every subsequent deploys, only the changes you make on top of the base image are downloaded.
The SSH server, so that you can easily login to your container to inspect or administer things. Password and challenge-response authentication are disabled by default. Only key authentication is allowed.
A build system, git, and development headers for many popular libraries, so that the most popular Ruby, Python and Node.js native extensions can be compiled without problems.