Improving speed of docker builds with two Gemfiles

.dockerignore

ansible
vendor/cache
log/*
tmp/cache/*
test

.ruby-version

2.6.4

Dockerfile

FROM ruby:2.6.4

RUN apt-get update -qq && apt-get install -y build-essential libpq-dev nodejs vim

ENV PROJECT_ROOT /usr/src/app

RUN mkdir -p $PROJECT_ROOT
WORKDIR $PROJECT_ROOT

RUN gem install bundler:2.0.2

COPY .ruby-version Gemfile-native Gemfile-native.lock ./
RUN bundle install --without development test --gemfile=Gemfile-native

COPY Gemfile Gemfile.lock ./
RUN bundle install --without development test

COPY . .

CMD bin/rails s

Gemfile

# frozen_string_literal: true

ruby File.read('.ruby-version').strip
instance_eval File.read('Gemfile-native')

# Bundle edge Rails instead: gem 'rails', github: 'rails/rails'
gem 'rails', '~> 6.0.2', '>= 6.0.2.2'
# Use postgresql as the database for Active Record
gem 'pg', '>= 0.18', '< 2.0'
# Use Puma as the app server
gem 'puma', '~> 4.1'
# Use SCSS for stylesheets
gem 'sass-rails', '>= 6'
# Build JSON APIs with ease. Read more: https://github.com/rails/jbuilder
gem 'jbuilder', '~> 2.7'
# Use Active Model has_secure_password
# gem 'bcrypt', '~> 3.1.7'

# Use Active Storage variant
# gem 'image_processing', '~> 1.2'
gem 'haml-rails'

# Reduces boot times through caching; required in config/boot.rb
gem 'bootsnap', '>= 1.4.2', require: false

group :development, :test do
  gem 'dotenv-rails'
  gem 'pry-byebug'
end

group :development do
  gem 'listen', '>= 3.0.5', '< 3.2'
  gem 'web-console', '>= 3.3.0'
  gem 'spring'
  gem 'spring-watcher-listen', '~> 2.0.0'
end

group :test do
  gem 'minitest-power_assert'
end

gem 'airbrake'
gem 'aws-sdk-s3'
gem 'dry-container'
gem 'haml'
gem 'metainspector'
gem 'oga'
gem 'persey'
gem 'redcarpet'
gem 'rubocop'
gem 'shufflino', '>= 0.3.2'
gem 'meta-tags'
gem 'rouge'

Gemfile-native

source 'https://rubygems.org'
git_source(:github) { |repo| "https://github.com/#{repo}.git" }

ruby File.read('.ruby-version').strip

gem 'sassc', '2.2.1'
gem 'nokogiri', '1.10.9'

Makefile

bundle:
	bundle install
	
build:
	docker build -t docker-experiment:two-gemfiles -f Dockerfile .
	
inspect:
	docker images | head -n 2