anchor.diff:  
   RUN DEBIAN_FRONTEND=noninteractive apt-get -y install wget
   RUN DEBIAN_FRONTEND=noninteractive apt-get -y install build-essential libpq-dev
  -
  -
  -# ------------------------------------------------------
  -# --- Pre-installed but not through apt-get
  -
  -# install Ruby from source
  -#  from source: mainly because of GEM native extensions,
  -#  this is the most reliable way to use Ruby no Ubuntu if GEM native extensions are required
  -RUN DEBIAN_FRONTEND=noninteractive apt-get -y install build-essential zlib1g-dev libssl-dev libreadline6-dev libyaml-dev
  -RUN wget -q http://cache.ruby-lang.org/pub/ruby/ruby-2.3.1.tar.gz
  -RUN tar -xvzf ruby-2.3.1.tar.gz
  -RUN cd ruby-2.3.1 && ./configure --prefix=/usr/local && make && make install
  -# cleanup
  -RUN rm -rf ruby-2.3.1
  -RUN rm ruby-2.3.1.tar.gz
  -
  -RUN gem install bundler --no-document
  +# RUN DEBIAN_FRONTEND=noninteractive apt-get -y install build-essential zlib1g-dev libssl-dev libreadline6-dev libyaml-dev
   
   
   # ------------------------------------------------------
  
positive.code: COPY Gemfile Gemfile
  COPY Gemfile.lock Gemfile.lock
  # RUN gem install bundler --version "=1.10.0" && bundle install --jobs 20 --retry 5
  RUN gem install bundler && bundle install --jobs 20 --retry 5
  
  COPY . /app
  
negative.code: FROM ruby:2.3.1
  
  
  # ------------------------------------------------------
  