Solution
app.run(host='0.0.0.0', port=8080)
$ flask run
* Running on http://127.0.0.1:5000/
# This is used when running locally only. When deploying to Google App
# Engine, a webserver process such as Gunicorn will serve the app. This
# can be configured by adding an `entrypoint` to app.yaml.
# Flask's development server will automatically serve static files in
# the "static" directory. See:
# http://flask.pocoo.org/docs/1.0/quickstart/#static-files. Once deployed,
# App Engine itself will serve those files as configured in app.yaml.
app.run(host='127.0.0.1', port=8080, debug=True)