Tableau sign in error when sending request to deployed service at Cloud Run

I've developed a custom provisioning service for Tableau Server using a Flask service at Google Cloud Run. I use a docker container to deploy my service into Google.

The service works fine when I run the container locally with postman, however, I get the error below when I send the request to the deployed service at Cloud Run.

<title>requests.exceptions.ConnectionError: HTTPConnectionPool(host='<mytableauserver.com>', port=80): Max retries
        exceeded with url: /api/2.4/serverInfo (Caused by NewConnectionError('&lt;urllib3.connection.HTTPConnection
        object at 0x3ed0efc890d0&gt;: Failed to establish a new connection: [Errno 111] Connection refused')) //
        Werkzeug Debugger</title>

app.py:

import os
import logging
from flask import Flask, jsonify, request
import provisioning

# Change the format of messages logged to Stackdriver
logging.basicConfig(format='%(message)s', level=logging.INFO)

app = Flask(__name__)

@app.route('/api/adduser', methods=['POST'])
def add():
    return jsonify(provisioning.add(request.json))

@app.route('/api/removeuser', methods=['POST'])
def remove():
    return jsonify(provisioning.remove(request.json))

@app.route('/', methods=['GET'])
def hello():
    return jsonify({'message' : 'Hello API'}) #Test Endpoint

if __name__ == "__main__":
    app.run(debug=True, host='0.0.0.0', port=int(os.environ.get('PORT', 80)))

provisioning.py:

import sys
import json
import tableauserverclient as TSC
import re


def sign_in(sitename):
    tableau_auth = TSC.PersonalAccessTokenAuth(
        token_name='<token_name>', personal_access_token='<token>', site_id=sitename)
    server = TSC.Server(
        server_address='<http://mytableauserver.com>', use_server_version=True)
    #server.add_http_options({'verify': False})
    server.auth.sign_in(auth_req=tableau_auth)
    return server  

def add(json_request):
    site = json_request["sitename"])
    session = sign_in(site) # Here is where the process fails
    site_role = json_request["rolename"]
    user = add_user_to_site(session, json_request, site_role)
    add_user_to_group(session, user, json_request)
    return json.dumps(json_request)

more code...

Dockerfile:

FROM python:3.8

ADD app.py /
ADD provisioning.py /

RUN pip install tableauserverclient
RUN pip install Flask

CMD [ "python", "./app.py" ]

What seems odd is the fact that I can sign into the server locally at the first try (I guess), but when try to reach the Cloud Run, it seems to be try multiple times until it fails. I use same code in all the containers.