Add NODE_ENV ARG to onbuild template

It's currently impossible to prevent npm installing devDependencies when using the node onbuild images. Adding a NODE_ENV ARG enables `docker build --build-arg NODE_ENV=production .`, which is the equivalent of `npm install --production`. Providing the `--build-arg` is optional, so this is backwards-compatible.

Dockerfile-onbuild.template

@@ -1,5 +1,7 @@
 FROM node:0.0.0
 
+ARG NODE_ENV
+
 RUN mkdir -p /usr/src/app
 WORKDIR /usr/src/app