relative_url_root の設定
参考：
Ruby on Railsのアプリをサブディレクトリで動かす場合のメモ – Qiita
Rails with Relative URL Root | Quickhack Diary
What is the replacement for ActionController::Base.relative_url_root? – Stack Overflow
参考：
Ruby on Railsのアプリをサブディレクトリで動かす場合のメモ – Qiita
Rails with Relative URL Root | Quickhack Diary
What is the replacement for ActionController::Base.relative_url_root? – Stack Overflow