Question: The official Docker documentation mentions that I need to run `docker rm -v containername` to specifically remove a data volume. But what do you do if you already removed all the containers referencing the specific data volume ?

Answer: 
Clean up commands: as of docker 1.13 introduces clean-up commands. To remove all unused containers, images, networks and volumes:

`docker system prune`

or individually:

`docker container prune`

`docker image prune`

`docker network prune`

`docker volume prune`
Use command 

```
docker volume ls -qf dangling=true | xargs -r docker volume rm

```

to clear dangling volumes from docker 1.9 and above.