Build both python 3.5 and 2.7 images from the same env

environments/python/README.md

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+# Fission: Python Environment
+
+This is the Python environment for Fission.
+
+It's a Docker image containing a Python 3.5 runtime, along with a
+dynamic loader.  A few common dependencies are included in the
+requirements.txt file.
+
+## Customizing this image
+
+To add package dependencies, edit requirements.txt to add what you
+need, and rebuild this image (instructions below).
+
+You also may want to customize what's available to the function in its
+request context.  You can do this by editing server.py (see the
+comment in that file about customizing request context).
+
+## Rebuilding and pushing the image
+
+You'll need access to a Docker registry to push the image: you can
+sign up for Docker hub at hub.docker.com, or use registries from
+gcr.io, quay.io, etc.  Let's assume you're using a docker hub account
+called USER.  Build and push the image to the the registry:
+
+```
+   docker build -t USER/python-env . && docker push USER/python-env
+```
+
+## Using the image in fission
+
+You can add this customized image to fission with "fission env
+create":
+
+```
+   fission env create --name python --image USER/python-env
+```
+
+Or, if you already have an environment, you can update its image:
+
+```
+   fission env update --name python --image USER/python-env   
+```
+
+After this, fission functions that have the env parameter set to the
+same environment name as this command will use this environment.