support dockerfiles without expose command. closes #1083

tests/apps/dockerfile-noexpose/Dockerfile

@@ -0,0 +1,13 @@
+FROM ubuntu:trusty
+ENV LC_ALL C
+ENV DEBIAN_FRONTEND noninteractive
+ENV DEBCONF_NONINTERACTIVE_SEEN true
+
+RUN apt-get install -y software-properties-common && add-apt-repository ppa:chris-lea/node.js && apt-get update
+RUN apt-get install -y build-essential curl postgresql-client-9.3 nodejs git
+
+COPY . /app
+WORKDIR /app
+RUN npm install
+
+CMD npm start