thanks @vvanpo
here’s what worked for me:
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'