Read asset host from ActionController::Base instead of application config

changelogs/unreleased/dm-favicon-asset-host.yml

+---
+title: Always serve favicon from main GitLab domain so that CI badge can be drawn
+  over it
+merge_request:
+author:
+type: fixed

lib/gitlab/favicon.rb

@@ -38,7 +38,8 @@ module Gitlab
       # we only want to create full urls when there's a different asset_host
       # configured.
       def host
-        if Gitlab::Application.config.asset_host.nil? || Gitlab::Application.config.asset_host == Gitlab.config.gitlab.base_url
+        asset_host = ActionController::Base.asset_host
+        if asset_host.nil? || asset_host == Gitlab.config.gitlab.base_url
           nil
         else
           Gitlab.config.gitlab.base_url

spec/lib/gitlab/favicon_spec.rb

@@ -32,7 +32,7 @@ RSpec.describe Gitlab::Favicon, :request_store do
       end
 
       it 'returns a full url when the asset host is configured' do
-        allow(Gitlab::Application.config).to receive(:asset_host).and_return('http://assets.local')
+        allow(ActionController::Base).to receive(:asset_host).and_return('http://assets.local')
         expect(described_class.main).to match %r{^http://localhost/assets/favicon-(?:\h+).png$}
       end
     end