ImportError raised when running catnip example

[yis11178]

The following error is raised when I tried to run the catnip example under section Dockerfile:

Traceback (most recent call last):
  File "/usr/src/app/./app.py", line 1, in <module>
    from flask import Flask, render_template
  File "/usr/local/lib/python3.10/site-packages/flask/__init__.py", line 21, in <module>
    from .app import Flask, Request, Response
  File "/usr/local/lib/python3.10/site-packages/flask/app.py", line 34, in <module>
    from .sessions import SecureCookieSessionInterface
  File "/usr/local/lib/python3.10/site-packages/flask/sessions.py", line 14, in <module>
    from collections import MutableMapping
ImportError: cannot import name 'MutableMapping' from 'collections' (/usr/local/lib/python3.10/collections/__init__.py)

This can be solved by changing the following line from the sample Dockerfile

FROM python:3

into

FROM python:3.9

[54D]

See #313