Afaik even though there is docker container update it is restricted to cpu/ram/io modifications and the restart policy.
You will need to delete your container and create a new one without the volume. Since containers are ment to be disposable/ephemeral there is nothing wrong in deleting the old container and creating a new one without the volume.
Docker doesn’t support what you ask for - though, it would totaly make sense to modify volumes on a stopped container… If its possible to atach/detach networks, why shouldn’t the same be implemented for volumes…
Update: they implemented it for swarm services: https://docs.docker.com/engine/reference/commandline/service_update/#add-or-remove-mounts. Basicly, this will be the managed version of delete the current container and re-create a new one… though, with the difference that it is managed by the swarm service.