Adjusting Dockerfile to use Yarn for faster npm installs

Dockerfile

@@ -3,8 +3,9 @@ FROM ruby:2.2.4
 ENV RAILS_ENV=production
 
 RUN echo 'deb http://httpredir.debian.org/debian jessie-backports main contrib non-free' >> /etc/apt/sources.list
-RUN apt-get update -qq && apt-get install -y build-essential libpq-dev libxml2-dev libxslt1-dev nodejs nodejs-legacy npm ffmpeg && rm -rf /var/lib/apt/lists/*
-RUN npm install -g npm@3
+RUN curl -sL https://deb.nodesource.com/setup_4.x | bash -
+RUN apt-get update -qq && apt-get install -y build-essential libpq-dev libxml2-dev libxslt1-dev nodejs ffmpeg && rm -rf /var/lib/apt/lists/*
+RUN npm install -g npm@3 && npm install -g yarn
 RUN mkdir /mastodon
 
 WORKDIR /mastodon
@@ -14,7 +15,8 @@ ADD Gemfile.lock /mastodon/Gemfile.lock
 RUN bundle install --deployment --without test development
 
 ADD package.json /mastodon/package.json
-RUN npm install
+ADD yarn.lock /mastodon/yarn.lock
+RUN yarn
 
 ADD . /mastodon