Document building. Add Dockerfile. Reference unofficial Docker build in README.

Author: Brian Cottingham

Dockerfile

@@ -0,0 +1,12 @@
+FROM node:10
+
+RUN npm install yarn
+
+WORKDIR /app
+ADD package.json /app/package.json
+ADD yarn.lock /app/yarn.lock
+RUN yarn install
+
+ADD . /app
+
+CMD npm start

README.md

@@ -10,4 +10,13 @@ Status](https://travis-ci.org/technoboy10/crossorigin.me.svg?branch=v2)](https:/
 
 crossorigin.me is a CORS proxy. It lets developers (like you!) access resources from other sites that don't have CORS enabled on their server. See [the website](https://crossorigin.me) for more information.
 
+# Building
 
+```
+npm install -g yarn
+yarn install
+npm start
+```
+
+# Docker
+An unofficial Docker image is available at `spiffytech/crossorigin.me`