build_number.rb

# /lib/custom_middleware/build_number.rb
module CustomMiddleware
  class BuildNumber
    def initialize(app)
      @app = app
    end

    def call(env)
      status, headers, body = @app.call(env)
      headers['X-Build-Number'] = Rails.configuration.build_number
      [status, headers, body]
    end
  end
end

# /config/application.rb
config.middleware.use CustomMiddleware::BuildNumber