Use the yarn pacakge manager for javascript

[Yarn](https://yarnpkg.com/) is a drop in replacement for npm.

We should use it because:

* It's faster
* It uses a lockfile, making the builds more deterministic.

webui/Dockerfile

@@ -1,13 +1,15 @@
-FROM node:6.3.0
+FROM node:6.9.1
 
 ENV WEBUI_DIR /src/webui
 RUN mkdir -p $WEBUI_DIR
 
 COPY package.json $WEBUI_DIR/
+COPY yarn.lock $WEBUI_DIR/
 
 WORKDIR $WEBUI_DIR
 RUN npm set progress=false
-RUN npm install --quiet
+RUN npm install --quiet --global yarn@0.16.1
+RUN yarn install
 
 COPY . $WEBUI_DIR/
 

webui/readme.md

@@ -19,12 +19,12 @@ make generate-webui  # Generate static contents in `traefik/static/` folder.
 
 ## How to build (only for frontends developer)
 
-- prerequisite: [Node 4+ and NPM 3+](https://nodejs.org)
+- prerequisite: [Node 4+](https://nodejs.org) [yarn](https://yarnpkg.com/)
 
 - Go to the directory `webui`
 
 - To install dependencies, execute the following commands:
-  - `npm install`
+  - `yarn install`
 
 - Build static Web UI, execute the following command:
   - `gulp`
@@ -62,6 +62,7 @@ make generate-webui  # Generate static contents in `traefik/static/` folder.
 ## Libraries
 
 - [Node](https://nodejs.org)
+- [Yarn](https://yarnpkg.com/)
 - [Generator FountainJS](https://github.com/FountainJS/generator-fountain-webapp)
 - [Webpack](https://github.com/webpack/webpack)
 - [AngularJS](https://docs.angularjs.org/api)