Document less common Python envt. variables

These are PYTHONUNBUFFERED and PYTHONDONTWRITEBYTECODE

Signed-off-by: Philippe Ombredanne <pombredanne@nexb.com>

Author: pombredanne

Dockerfile

@@ -24,7 +24,9 @@ FROM --platform=linux/amd64 python:3.9
 
 WORKDIR /app
 
+# Python settings: Force unbuffered stdout and stderr (i.e. they are flushed to terminal immediately)
 ENV PYTHONUNBUFFERED 1
+# Python settings: do not write pyc files
 ENV PYTHONDONTWRITEBYTECODE 1
 
 # OS requirements as per