After upgrading to Ruby 2.5.3, we switched URI.join in favor of Gitlab::Utils.append_path. However, ActionController::Base.helpers.image_path can return a full URL if a CDN host is present.
Rails provides a way to generate the full URL using the asset path, but
that doesn't appear to work because request is nil`.
Revert to the previous behavior to handle CDNs and relative URLs.