Fix #1564: Use Procfile for process types in Dockerfile apps

* Document new Procfile feature
* Add and update tests
* Fix a typo in generate_scale_file (ref #1923)

tests/apps/dockerfile-procfile/worker.js

@@ -0,0 +1,6 @@
+function worker() {
+  console.log('sleeping for 60 seconds');
+  setTimeout(worker, 60 * 1000);
+}
+
+worker();