Can only remove volume after running system prune --volumes

[sbriels]

After quite some trial and error I noticed I can only remove volumes after I ran "docker sytem prune --volumes". Is this the intended behaviour? If so it would be good to put this in the readme?

Ran steps:

1. installing the plugin as: docker plugin install --grant-all-permissions vieux/sshfs sshkey.source=/root/.ssh/
2. setting up public/private key for root (sudo -i , then ssh-keygen -b 4096 -t rsa )
3. setting up the volume: docker volume create -d vieux/sshfs -o sshcmd=user@192.168.1.1:/home/user/ sshvolume
4. using the volume: docker run -it -v sshvolume:/data ubuntu
5. removing the volume by "docker volume rm sshvolume" gives "Error response from daemon: remove sshvolume: volume is in use"
6. run "docker system prune --volumes"
7. remove by "docker volume rm sshvolume" (succes!)