Setup NPM with an empty package.json file

Note that, even if we're excluding the `node_modules/` folder from
version control, we aren't adding it to Capistrano's shared folders
because, when `node_modules` is a symbolic link, NPM removes it when
running `npm install`.

Dockerfile

@@ -51,6 +51,9 @@ RUN curl -sL https://github.com/nodenv/node-build/archive/master.tar.gz | tar xz
 COPY Gemfile* ./
 RUN bundle install
 
+COPY package* ./
+RUN npm install
+
 # Copy the Rails application into place
 COPY . .