Decouple config.asset_host assignment from rails:

lib/sprockets/railtie.rb

@@ -201,6 +201,11 @@ def self.build_manifest(app)
       Sprockets::Manifest.new(app.assets, path, config.assets.manifest)
     end
 
+    config.after_initialize do |app|
+      config.action_controller.asset_host        ||= app.config.asset_host
+      config.action_controller.relative_url_root ||= app.config.relative_url_root
+    end
+
     config.after_initialize do |app|
       config = app.config
 

test/test_railtie.rb

@@ -410,4 +410,18 @@ def test_quiet_assets_inserts_middleware
     assert middleware.include?(Sprockets::Rails::QuietAssets)
     assert middleware.each_cons(2).include?([Sprockets::Rails::QuietAssets, Rails::Rack::Logger])
   end
+
+  def test_config_asset_host_can_be_set_inside_an_initializer
+    app.configure do
+      initializer 'asset_host_and_relative_url_root' do |app|
+        app.config.asset_host        = 'https://example.com'
+        app.config.relative_url_root = '/myassets'
+      end
+    end
+    app.initialize!
+    context_class = Sprockets::Railtie.build_environment(app).context_class
+
+    assert_equal 'https://example.com', context_class.config.asset_host
+    assert_equal '/myassets',           context_class.config.relative_url_root
+  end
 end