Add Docker badge

Author: vishesh

README.md

@@ -2,7 +2,8 @@ Racketscript Playground
 =======================
 
 [![MIT licensed](https://img.shields.io/badge/license-MIT-blue.svg)](COPYING.md)
-[![Try Online](https://img.shields.io/badge/try_it-online!-ff9900.svg)]( http://play.racketscript.org)
+[![Docker](https://img.shields.io/docker/cloud/automated/vishesh/racketscript-playground)](https://hub.docker.com/r/vishesh/racketscript-playground)
+[![Try Online](https://img.shields.io/badge/try_it-online!-ff9900.svg)](http://play.racketscript.org)
 
 Playground for [RacketScript](https://github.com/vishesh/racketscript). 
 Both server-side and client-side code is written in RacketScript.