Set SCRIPT_NAME inside Rack middleware

config.ru

require ::File.expand_path('../config/environment', __FILE__)

class ScriptName
  def initialize(app)
    @app = app
  end

  def call(env)
    env['SCRIPT_NAME'] += env['HTTP_FRONTEND_URI'].to_s if !Rails.env.development?
    @app.call(env)
  end
end

use ScriptName

run Rails.application