Why Flask logger does not log in docker when using UWSGI in front?

I have a Flask application inside of Docker that was logging into docker logs when it was running without UWSGI in front. Now I have used UWSGI with the config below to run my application inside of Docker:

[uwsgi]

master = true

processes = 5
threads = 2

socket = 127.0.0.1:3031
chmod-socket = 664
stats=0.0.0.0:30310

chdir = /etc/fantas

uid = root
gid = root

wsgi-file=uwsgi_fantas.py
callable=app

vacuum = true

The uwsgi_fantas.py file contains:

from fantas.fantas_app import FantasApp

app = FantasApp().setup()

The setup method returns app:

from flask_restful import Api
from fantas import app

class FantasApp(object):
    def setup(self):
        api = Api(app)
        api.add_resource(Token, '/users')

        return app

Finally the part that initiates the Flask framework is inside of __init__.py in the root directory of project:

from flask import Flask
import logging

app = Flask(__name__)

s_handler = logging.StreamHandler()
s_handler.setLevel(logging.DEBUG)
app.logger.addHandler(s_handler)

As UWSGI works directly with the app object I have configured logger inside of __init__.py, but the problem is that it does not log anything into Docker when it is run, it just logs UWSGI requests.

What is wrong in the process of app.logger configuration?

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EDIT-1: I set app.logger.setLevel(logging.DEBUG) and it seems that Flask logs into Docker successfully. The weird part is that it logs 3 times! I removed all logger configs and handlers and just used:

app.logger.setLevel(logging.DEBUG)

But now it logs 2 times:

proj_fantas.1.huagnqqpzo1n@linuxkit-025000000001    | [2018-07-13 07:02:38,008] DEBUG in token: [Token] authenticating user...
proj_fantas.1.huagnqqpzo1n@linuxkit-025000000001    | DEBUG:flask.app:[Token] authenticating user...

Why it is like that?

---

EDIT-2:

The output of app.logger.handlers is [<logging.StreamHandler object at 0x7f0f430ca8d0>]. It just shows the StreamHandler that I initialized earlier, nothing more.

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