MQTT (Message Queuing Telemetry Transport) is an open network protocol for transmitting messages between devices. An MQTT broker, like the open source Mosquitto server, is a simple solution to receive data from SmartHome devices or to control them .As an example, I use MQTT with an ESP32 microcontroller to have it send values to the MQTT broker, which can be retrieved via the MQTT integration in HomeAssistant. In addition, I use the MQTTBroker to communicate my Zigbee devices via Zigbee2MQT, see: Home-Assistant Docker Conbee 2 and Zigbee2MQTT / deCONZ 

Those who have Docker and Docker Compose installed on their system can start the Mosquitto MQTT broker after creating the following file, for now without the config folder so that we can assign a password for MQTT:

docker-compose.yml:

version: "2"
 services:
  mosquitto:
    image: eclipse-mosquitto
    container_name: mqtt
    restart: always
    volumes:
      - ./mosquitto/data:/mosquitto/data
      - ./mosquitto/log:/mosquitto/log
    ports:
      - "1883:1883"
      - "9001:9001"

docker-compose up starts the container:

docker-compose up

Attention the folder /mosquitto/config and therefore the config file: mosquitto.conf is still missing in the docker-compose file for now.This is initially created at startup and is only located inside the container, so all settings would be lost when the container is restarted. At this point, the container already works with the default settings, but allows all devices to communicate with the broker without a password:

c:\temp\mosquitto>docker-compose up
Recreating mqtt ... done
Attaching to mqtt
mqtt         | 1642331896: mosquitto version 2.0.14 starting
mqtt         | 1642331896: Config loaded from /mosquitto/config/mosquitto.conf.
mqtt         | 1642331896: Starting in local only mode. Connections will only be possible from clients running on this machine.
mqtt         | 1642331896: Create a configuration file which defines a listener to allow remote access.
mqtt         | 1642331896: For more details see https://mosquitto.org/documentation/authentication-methods/
mqtt         | 1642331896: Opening ipv4 listen socket on port 1883.
mqtt         | 1642331896: Opening ipv6 listen socket on port 1883.
mqtt         | 1642331896: Error: Address not available
mqtt         | 1642331896: mosquitto version 2.0.14 running

So that we can assign a password for MQTT and the settings remain after restarting the container, I connected into the container and created a password file in it:

c:\temp\mosquitto> docker exec -it mqtt sh
/ # mosquitto_passwd -b -c passwd mqtt myMQTTPassword
/ # cat passwd
mqtt:xxxx

The contents of the output mqtt:??? can then be put into the passwd file in the /config folder:

Then another new file with the configuration: mosquitto.conf with the following content:

port 1883
listener 9001
protocol websockets
persistence true
persistence_location /mosquitto/data
allow_anonymous false
password_file /mosquitto/config/passwd

Afterwards, the Docker container can be stopped, the /mosquitto/config folder can be added and started again:

Adjustments to the docker-compose.yml file: Addition of /config:

version: "2"
services:
  mosquitto:
    image: eclipse-mosquitto
    container_name: mqtt
    restart: always
    volumes:
      - ./mosquitto/config:/mosquitto/config
      - ./mosquitto/data:/mosquitto/data
      - ./mosquitto/log:/mosquitto/log
    ports:
      - "1883:1883"
      - "9001:9001"

Restarting the container uses the created configuration and the assigned password:

docker-compose up -d

A look into the output of the container (docker logs) shows us the successful start:

c:\temp\mosquitto>docker logs mqtt -f
1642333000: mosquitto version 2.0.14 starting
1642333000: Config loaded from /mosquitto/config/mosquitto.conf.
1642333000: Opening websockets listen socket on port 9001.
1642333000: Opening ipv4 listen socket on port 1883.
1642333000: Opening ipv6 listen socket on port 1883.
1642333000: mosquitto version 2.0.14 running

see, Home Assistant Docker Conbee 2 and Zigbee2MQTT / deCONZ

Display of connections: Logs

In the Docker logs, the connection of individual devices can be observed:

docker logs mqtt -f

1641494340: New connection from 172.22.0.4:44135 on port 1883.
1641494340: New client connected from 172.22.0.4:44135 as xxxxxx (p2, c1, k60, u'mqtt').
1641494608: Client xxx closed its connection.
1641496053: Saving in-memory database to /mosquitto/data/mosquitto.db.