Move image_optim to the docs bundle group

Only the scrapers optimize images since the spritesheets went through
cwebp, so the app installed five megabytes of binaries it never ran.

Author: simon04

Gemfile

@@ -13,8 +13,6 @@ group :app do
   gem 'chunky_png'
   gem 'erubi'
   gem 'dartsass-sprockets'
-  gem 'image_optim_pack', platforms: :ruby
-  gem 'image_optim'
   gem 'puma'
   gem 'rack'
   gem 'rss'
@@ -35,6 +33,8 @@ group :development do
 end
 
 group :docs do
+  gem 'image_optim_pack', platforms: :ruby
+  gem 'image_optim'
   gem 'kramdown'
   gem 'kramdown-parser-gfm'
   gem 'redcarpet'

lib/docs/filters/core/images.rb

@@ -1,7 +1,6 @@
 # frozen_string_literal: true
 
 require 'base64'
-require 'image_optim'
 
 module Docs
   class ImagesFilter < Filter
@@ -21,6 +20,9 @@ class ImagesFilter < Filter
     CWEBP_LOSSY_COMMAND = %W(cwebp -quiet -q #{JPEG_QUALITY} -m 6 -sharp_yuv -metadata none -o - -- -).freeze
     GIF2WEBP_COMMAND = %w(gif2webp -quiet -m 6 -metadata none -o - -- -).freeze
 
+    # image_optim belongs to the docs bundle group, which the app leaves out;
+    # `Bundler.require :default, :docs` in docs.rb loads it for the scrapers,
+    # which are the only ones optimizing anything.
     def self.optimize_image_data(data)
       @image_optim ||= ImageOptim.new
       @image_optim.optimize_image_data(data)