感谢 @vvanpo
以下是我成功的做法：
RAILS_ENV=production bundle exec discourse remap '/uploads' '/<subfolder>/uploads'
RAILS_ENV=production bundle exec discourse remap '/user_avatar/<domain>' '/<subfolder>/user_avatar/<domain>'
RAILS_ENV=production bundle exec discourse remap '/images' '/<subfolder>/images'
RAILS_ENV=production bundle exec discourse remap '/plugins' '/<subfolder>/plugins'