Python mqtt client in a test docker container connecting to mqtt broker docker container

i have a test container containing a python script unit test to test the connection to a mqtt broker. The mqtt broker is another docker container using eclipse-mosquitto:latest.

I have also created a docker network.

Below are the docker statements:

docker network create test_network
docker run -d -p 21883:1883 --name=gatewaymqtt --network=test_network -v "$PWD/work/mosquitto/config/mosquitto.conf:/mosquitto/config/mosquitto.conf" -v "$PWD/work/mosquitto/data:/mosquitto/data" -v "$PWD/work/mosquitto/log:/mosquitto/log" eclipse-mosquitto:latest

docker build -t test -f ./Dockerfile-test .
docker run --network=test_network test

Inside the test container it is running this python unit test:

class TestClient(unittest.TestCase):

def test_mqtt_connect(self):
    log = logging.getLogger("test_mqtt_connect")
    # The callback for when the client receives a CONNACK response from the server.
    def on_connect(client, userdata, flags, rc):
        log.debug("Connected with result code "+str(rc))

        # Subscribing in on_connect() means that if we lose the connection and
        # reconnect then subscriptions will be renewed.
        #client.subscribe("$SYS/#")

    # The callback for when a PUBLISH message is received from the server.
    def on_message(client, userdata, msg):
        log.debug(msg.topic+" "+str(msg.payload))

    client = mqtt.Client()
    client.on_connect = on_connect
    client.on_message = on_message

    client.connect("gatewaymqtt ", 21883, 60)
    sleep(5)
    client.disconnect()

if __name__ == '__main__':
    unittest.main()

However, I do not see any incoming connection from the test container?

This is the log of the test container:

======================================================================
ERROR: test_mqtt_connect (__main__.TestClient)
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Traceback (most recent call last):
  File "test_socketio.py", line 69, in test_mqtt_connect
    client.connect("gatewaymqtt ", 21883, 60)
  File "/usr/local/lib/python3.7/site-packages/paho/mqtt/client.py", line 937, in connect
    return self.reconnect()
  File "/usr/local/lib/python3.7/site-packages/paho/mqtt/client.py", line 1071, in reconnect
    sock = self._create_socket_connection()
  File "/usr/local/lib/python3.7/site-packages/paho/mqtt/client.py", line 3522, in _create_socket_connection
    return socket.create_connection(addr, source_address=source, timeout=self._keepalive)
  File "/usr/local/lib/python3.7/socket.py", line 728, in create_connection
    raise err
  File "/usr/local/lib/python3.7/socket.py", line 716, in create_connection
    sock.connect(sa)
ConnectionRefusedError: [Errno 111] Connection refused

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