Use base ruby image in Dockerfile & fix watcher

It seems that the watcher will not work without the
--force-polling option.

Dockerfile

-FROM ubuntu:wily
+FROM ruby:2.2.3-onbuild
+
+RUN ln -s /usr/src/app /app # Deprecated
 
-RUN apt-get update
-RUN apt-get install -yq ruby ruby-dev build-essential git nodejs
-RUN gem install --no-ri --no-rdoc bundler
-ADD Gemfile /app/Gemfile
-ADD Gemfile.lock /app/Gemfile.lock
-RUN cd /app; bundle install
-ADD . /app
 EXPOSE 4567
-WORKDIR /app
-CMD ["bundle", "exec", "middleman", "server"]
+CMD ["bundle", "exec", "middleman", "server", "--force-polling"]