code: def recover_buildpack(app_folder):
      
      filepath = os.path.join(app_folder, '.buildpack')
      with open(filepath) as f:
          buildpack_picked = f.read()
      buildpack_picked = buildpack_picked.lstrip('/')
      buildpack_picked = buildpack_picked.rstrip('\n')
      buildpack_picked = os.path.join(os.getcwd(), buildpack_picked)
      return BuildPack(buildpack_picked)
docs: Given the path to an app folder where an app was just built, return a
      BuildPack object pointing to the dir for the buildpack used during the
      build.
  
      Relies on the builder.sh script storing the buildpack location in
      /.buildpack inside the container.