You may not need a bundler for your NPM library(cmdcolin.github.io)

  FROM ruby_with_node_and_python AS builder

  # Copy only what's needed for installing dependencies and other garbage (e.g. changed code files won't invalidate Docker cache)
  COPY ./src/.browserslistrc ./src/.ruby-version ./src/babel.config.js ./src/config.ru ./src/Gemfile ./src/Gemfile.lock ./src/yarn.lock ./src/package.json ./src/postcss.config.js ./src/Rakefile /app

  # install front end dependencies
  RUN yarn install
  # add code and run asset creation
  COPY ./src /app
  RUN bundle exec rake assets:clobber RAILS_ENV=production
  RUN bundle exec rake assets:precompile RAILS_ENV=production

  # install back end dependencies
  RUN bundle install

  # run tests (local SQLite, for example)
  RUN rails db:migrate RAILS_ENV=test
  RUN rails test RAILS_ENV=test

  # clean up files after build (so copying in later images doesn't drag garbage along; this could be a separate intermediate image too)
  RUN rm -rf src/node_modules
  # clean up files after test (so copying in later images doesn't drag garbage along; this could be a separate intermediate image too)
  RUN rm -rf src/log/*.log src/tmp/*

  FROM ruby_with_nothing_else AS runner

  # copy over application files
  COPY --from=builder /app /app

  FROM ruby_with_package_manager AS runner

  # we actually could not carry over the precompiled gems etc. because I'm stupid, so instead we now run the install again :(
  # well it's not like you need front end files here, but this doesn't really mean much
  COPY ./src/.browserslistrc ./src/.ruby-version ./src/babel.config.js ./src/config.ru ./src/Gemfile ./src/Gemfile.lock ./src/yarn.lock ./src/package.json ./src/postcss.config.js ./src/Rakefile /app

  # install back end dependencies
  RUN bundle install

  # copy over application files
  COPY --from=builder /app /app