Improve environment doc

environments/python3/README.md

@@ -8,11 +8,12 @@ requirements.txt file.
 
 ## Customizing this image
 
-Most commonly you might want to add package dependencies.  Edit
-requirements.txt to add what you need, and rebuild 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
-context.  You can do this by editing server.py.
+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
 

environments/python3/server.py

@@ -24,6 +24,8 @@ def f():
         print("Generic container: no requests supported")
         abort(500)
     #
+    # Customizing the request context
+    #
     # If you want to pass something to the function, you can add it to 'g':
     #   g.myKey = myValue
     # And the user func can then access that (after doing a "from flask import g").