Replace the version-specific Dockerfiles with one staged Dockerfile

Gemfile

@@ -28,20 +28,17 @@ gem 'sanitize', '~> 6.1'
 gem 'tracks-chartjs-ror'
 gem 'will_paginate'
 
-group :layout do
-  gem 'listen'
-  gem 'tolk', '~> 5.0.1'
-end
-
-# See https://github.com/sstephenson/execjs#readme for more supported runtimes
-gem 'mini_racer', group: :therubyracer
-
 # Use --without <group> argument to skip unnecessary drivers
 gem 'sqlite3', group: :sqlite
 gem 'mysql2', '~> 0.5.6', group: :mysql
 gem 'pg', '~> 1.5.6', group: :postgresql
 
-group :development do
+group :assets, :optional => true do
+  gem 'listen'
+  gem 'tolk', '~> 5.0.1'
+end
+
+group :development, :optional => true do
   gem 'spring', '~> 4'
   gem 'yard'
 
@@ -52,22 +49,25 @@ group :development do
   gem 'i18n-tasks', '~> 1.0.14'
 end
 
-group :development, :test do
+group :development, :test, :optional => true do
   gem 'byebug'
-  gem 'rubocop', '~> 1.65', require: false
+  gem 'rubocop', '~> 1.65'
+  gem 'mini_racer'
 end
 
-group :test do
+group :test, :optional => true do
   # get test coverage info on codeclimate
-  gem 'codeclimate-test-reporter', '1.0.9', group: :test, require: nil
+  gem 'codeclimate-test-reporter', '1.0.9'
   gem 'database_cleaner', '~> 1'
   gem 'factory_bot_rails'
   gem 'minitest-stub-const'
-  gem 'mocha', :require => false
+  gem 'mocha'
   gem 'rails-controller-testing'
   gem 'rails-dom-testing', '~> 2.2.0'
   gem 'rspec-expectations'
   gem 'simplecov'
 end
 
-gem 'stripe', "~> 5.24.0", group: :stripe
+group :stripe, :optional => true do
+  gem 'stripe', "~> 5.24.0"
+end