Persistence with Mosquitto

[Narsskrarc]

It seems to be quite simple to add persistence to mosquitto. Simply add
persistence true
persistence_location /mosquitto/data
to data/mqtt/config/mosquitto.conf.

But - that doesn't work.
The reason is, that /mosquitto/data isn't mounted to data/mqtt/data, as one could expect.
You have to add

mqtt:
    volumes:
      - ./data/mqtt/data:/mosquitto/data

explicitly to the docker-compose.yml. Only then the persistence is really persistent.