Support Tailwind CSS in core dummy app

This allows running the core dummy app with the Tailwind required for
setting sails towards the new frontend.

core/lib/spree/testing_support/dummy_app/rake_tasks.rb

@@ -19,6 +19,41 @@ def initialize(gem_root:, lib_name:)
   end
 end
 
+if Object.const_defined?(:SolidusAdmin)
+  namespace :solidus_admin do
+    namespace :tailwindcss do
+      desc 'Build Tailwind CSS'
+      task build: :dummy_environment do
+        require "solidus_admin"
+        require "tailwindcss/commands"
+
+        tailwindcss = Tailwindcss::Commands.executable
+
+        tailwindcss_command = [
+          tailwindcss,
+          "--config", SolidusAdmin::Engine.root.join("config/tailwind.config.js"),
+          "--input", SolidusAdmin::Engine.root.join("app/assets/stylesheets/solidus_admin/application.tailwind.css"),
+          "--output", DummyApp::Application.root.join("assets/builds/solidus_admin/tailwind.css"),
+        ]
+
+        sh tailwindcss_command.shelljoin
+      end
+    end
+  end
+
+  # Attach Tailwind CSS build to other tasks.
+  %w[
+    assets:precompile
+    test:prepare
+    spec:prepare
+    db:test:prepare
+  ].each do |task_name|
+    next unless Rake::Task.task_defined?(task_name)
+
+    Rake::Task[task_name].enhance(['solidus_admin:tailwindcss:build'])
+  end
+end
+
 namespace :db do
   desc "Drops the test database"
   task drop: :dummy_environment do