python-socketio: socketio.exceptions.ConnectionError: OPEN packet not returned by server

I want to run my Socket-IO Client with Docker, but I get the following error message when trying to connect to my Socket-IO Server (written in JavaScript).

Traceback (most recent call last):
  File "client.py", line 14, in <module>
    sio.connect('http://' + server_address + ':3000')
  File "/usr/local/lib/python3.8/dist-packages/socketio/client.py", line 280, in connect
    raise exceptions.ConnectionError(exc.args[0]) from None
socketio.exceptions.ConnectionError: OPEN packet not returned by server

Socket-IO Client:

import socketio
import os, sys

# Connect to socket io server
sio = socketio.Client()

server_address = os.environ.get('SERVER_ADDRESS')
if server_address:
    print('http://' + server_address + ':3000')
    sio.connect('http://' + server_address + ':3000')
else:
    sio.connect('http://<default ip>:3000')

Dockerfile:

FROM ubuntu:latest

ARG SERVER_ADDRESS
ENV SERVER_ADDRESS $SERVER_ADDRESS

RUN apt-get update -y && apt-get install -y python3 python3-pip

RUN pip3 install pyserial python-socketio requests

COPY ./flir-config .

COPY ./find_serial_device.py .

CMD ["python3", "client.py"]