docs: Add information to README about Docker image & Docker Hub (#676)

closes #657

README.md

@@ -5,7 +5,7 @@
 
   [![Build Status](https://travis-ci.org/Rebilly/ReDoc.svg?branch=master)](https://travis-ci.org/Rebilly/ReDoc) [![Coverage Status](https://coveralls.io/repos/Rebilly/ReDoc/badge.svg?branch=master&service=github)](https://coveralls.io/github/Rebilly/ReDoc?branch=master) [![dependencies Status](https://david-dm.org/Rebilly/ReDoc/status.svg)](https://david-dm.org/Rebilly/ReDoc) [![devDependencies Status](https://david-dm.org/Rebilly/ReDoc/dev-status.svg)](https://david-dm.org/Rebilly/ReDoc#info=devDependencies) [![npm](http://img.shields.io/npm/v/redoc.svg)](https://www.npmjs.com/package/redoc) [![License](https://img.shields.io/npm/l/redoc.svg)](https://github.com/Rebilly/ReDoc/blob/master/LICENSE)
 
-  [![bundle size](http://img.badgesize.io/https://cdn.jsdelivr.net/npm/redoc/bundles/redoc.standalone.js?compression=gzip&max=300000)](https://cdn.jsdelivr.net/npm/redoc/bundles/redoc.standalone.js) [![npm](https://img.shields.io/npm/dm/redoc.svg)](https://www.npmjs.com/package/redoc) [![](https://data.jsdelivr.com/v1/package/npm/redoc/badge)](https://www.jsdelivr.com/package/npm/redoc)
+  [![bundle size](http://img.badgesize.io/https://cdn.jsdelivr.net/npm/redoc/bundles/redoc.standalone.js?compression=gzip&max=300000)](https://cdn.jsdelivr.net/npm/redoc/bundles/redoc.standalone.js) [![npm](https://img.shields.io/npm/dm/redoc.svg)](https://www.npmjs.com/package/redoc) [![](https://data.jsdelivr.com/v1/package/npm/redoc/badge)](https://www.jsdelivr.com/package/npm/redoc) [![Docker Build Status](https://img.shields.io/docker/build/redocly/redoc.svg)](https://hub.docker.com/r/redocly/redoc/)
 
 
 </div>
@@ -184,6 +184,17 @@ You can also specify `onLoaded` callback which will be called each time Redoc ha
 />
 ```
 
+## The Docker way
+
+ReDoc is available as pre-built Docker image in official [Docker Hub repository](https://hub.docker.com/r/redocly/redoc/). You may simply pull & run it:
+
+    docker pull redocly/redoc
+    docker run -p 80:8080 redocly/redoc
+
+Also you may rewrite some predefined environment variables defined in [Dockerfile](./Dockerfile). By default ReDoc starts with demo Petstore spec located at `http://petstore.swagger.io/v2/swagger.json`, but you may change this URL using environment variable `SPEC_URL`:
+
+    docker run -p 80:8080 -e SPEC_URL=https://api.example.com/openapi.json redocly/redoc
+
 ## ReDoc CLI
 
 [See here](https://github.com/Rebilly/ReDoc/blob/master/cli/README.md)