Persistence with Mosquitto

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.