support dockerfiles without expose command. closes #1083

tests/apps/dockerfile-noexpose/index.js

@@ -0,0 +1,13 @@
+var express = require('express')
+var app = express();
+
+app.set('port', (process.env.PORT || 5000))
+app.use(express.static(__dirname + '/public'))
+
+app.get('/', function(request, response) {
+  response.send('Hello World!')
+})
+
+app.listen(app.get('port'), function() {
+  console.log("Node app is running at localhost:" + app.get('port'))
+})