Use phusion/baseimage-docker as base image instead of vanilla Ubuntu

[phusion/baseimage-docker](https://github.com/phusion/baseimage-docker) is a Docker image tuned for usage within Docker containers. It is based on Ubuntu, but includes several attractive features:

- Adds ability to run multiple services with [runit](http://smarden.org/runit/)
- Adds hooks for easily running scripts during startup
- Centralizes environmental variable management
- Includes useful services like `logrotate`, `cron`, `ssh`, and `syslog-ng`

This pull request switches the base Docker image from `ubuntu` to `phusion/baseimage`.