Utilizing Docker's caching system

Dockerfile

# Choose the official Ruby 2.3.0 image as our starting point
FROM ruby:2.3.0

# Run updates for nokogiri and JS runtime
RUN apt-get update -qq && apt-get install -y build-essential libxml2-dev libxslt1-dev nodejs

# Cleanup
RUN apt-get clean && rm -rf /var/lib/apt/lists/* /tmp/* /var/tmp/*

# Set up working directory
RUN mkdir /app

# Copy Gemfiles and install gems
WORKDIR /tmp
COPY Gemfile Gemfile
COPY Gemfile.lock Gemfile.lock
RUN bundle install

# Everything up to here was cached. This includes
# the bundle install, unless the Gemfiles changed.

# Change back to app directory
WORKDIR /app
ADD . /app # optional if we mount local volumes to /app

# Start the server
CMD ["rails", "server", "-b", "0.0.0.0"]