Add ability to override any configs with env

Use environemnt variables to override any value in config.php

Also, locked version on the ubuntu image to something that actually
builds.

Dockerfile

@@ -1,4 +1,4 @@
-FROM ubuntu
+FROM ubuntu:14.04
 MAINTAINER Christian Lück <christian@lueck.tv>
 
 RUN DEBIAN_FRONTEND=noninteractive apt-get update && apt-get install -y \