Dockerfile: remove ADD
Dockerfile: remove ADD

ADD is not necessary since `google/nodejs-runtime` do `ONBUILD`
johnschimmel/appengine-nodejs-quickstart,jonparrott/appengine-nodejs-quickstart,jonparrott/appengine-nodejs-quickstart,johnschimmel/appengine-nodejs-quickstart,googlearchive/appengine-nodejs-quickstart,googlearchive/appengine-nodejs-quickstart,wujf/appengine-nodejs-quickstart