fix docker container rm -v flag description and docs to mention "anonymous" volumes only

[thaJeztah]

Need to double-check if this was by design, but docker rm -v (remove container including volumes attached to it) only removes anonymous volumes, but not named ones. However, the flag description does not mention this;

docker container rm --help

Usage:	docker rm [OPTIONS] CONTAINER [CONTAINER...]

Remove one or more containers

Options:
  -f, --force     Force the removal of a running container (uses SIGKILL)
  -l, --link      Remove the specified link
  -v, --volumes   Remove the volumes associated with the container

Steps to verify the current behavior:

docker run -d --name mycontainer1 -v /anonymous -v namedvol1:/namedvol nginx:alpine
docker run -d --name mycontainer2 -v /anonymous -v namedvol2:/namedvol nginx:alpine

docker volume ls

DRIVER              VOLUME NAME
local               547d032c5571241ec33b6064dcaef1116c646b8a7e590a18bc56da9a2a22ddab
local               402827d64bced57d80125e52606d6b394c651c138cb829587798b041357523c2
local               namedvol1
local               namedvol2

Removing mycontainer1 keeps all volumes associated with the container:

docker rm -f mycontainer1
docker volume ls
DRIVER              VOLUME NAME
local               547d032c5571241ec33b6064dcaef1116c646b8a7e590a18bc56da9a2a22ddab
local               402827d64bced57d80125e52606d6b394c651c138cb829587798b041357523c2
local               namedvol1
local               namedvol2

Whereas using the -v option removes the anonymous volume, but keeps the named volume:

docker rm -fv mycontainer2
docker volume ls
DRIVER              VOLUME NAME
local               547d032c5571241ec33b6064dcaef1116c646b8a7e590a18bc56da9a2a22ddab
local               namedvol1
local               namedvol2

[thaJeztah]

Note that docker run --rm (run and auto-remove after exiting) does always remove anonymous volumes

[thaJeztah]

/cc @ndeloof @cpuguy83

[cpuguy83]

It is intentional.
https://github.com/moby/moby/blob/ac058c1629dcf6912d800c08b8eb6a6c8b67427d/daemon/mounts.go#L34-L36

[thaJeztah]

Thanks! I thought it was, but know there has been some back-and-forth around this 👍