doc(README.md): add container badge to README

Author: Aaron Schlesinger

README.md

@@ -1,6 +1,6 @@
 # Deis Builder v2
 
-[![Build Status](https://travis-ci.org/deis/builder.svg?branch=master)](https://travis-ci.org/deis/builder) [![Go Report Card](http://goreportcard.com/badge/deis/builder)](http://goreportcard.com/report/deis/builder)
+[![Build Status](https://travis-ci.org/deis/builder.svg?branch=master)](https://travis-ci.org/deis/builder) [![Go Report Card](http://goreportcard.com/badge/deis/builder)](http://goreportcard.com/report/deis/builder) [![Docker Repository on Quay](https://quay.io/repository/deisci/builder/status "Docker Repository on Quay")](https://quay.io/repository/deisci/builder)
 
 Deis (pronounced DAY-iss) is an open source PaaS that makes it easy to deploy and manage
 applications on your own servers. Deis builds on [Kubernetes](http://kubernetes.io/) to provide