Make default node-env use alpine. List envs in documentation. (#354)

closes #319 

* Make default node-env use alpine, and support separate debian env. List envs in documentation.

* Copy ONBUILD base image commands into node Dockerfiles

* Copy more commands from onbuild image

* Tidy dockerfiles

examples/nodejs/README.md

@@ -2,12 +2,19 @@
 
 This directory contains several examples to get you started using Node.js with Fission.
 
+## Environment
+
 Before running any of these functions, make sure you have created a `nodejs` Fission environment:
 
 ```
 $ fission env create --name nodejs --image fission/node-env
 ```
 
+Note: The default `fission/node-env` image is based on Alpine, which is much smaller than the main Debian Node image (65MB vs 680MB) while still being suitable for most use cases.
+If you need to use the full Debian image use the `fission/node-env-debian` image instead.
+See the [official Node docker hub repo](https://hub.docker.com/_/node/) for considerations 
+relating to this choice.
+
 ## Function signature
 
 Every Node.js function has the same basic form: