docker serve: make Create idempotent to avoid "volume already exists" after restart

Docker may re-send Create requests for volumes that already exist,
especially after a plugin restart. Previously this returned
ErrVolumeExists which Docker surfaced as "volume name must be unique".

Now if a volume with the same name already exists, Create returns
success (no-op), matching the Docker volume plugin protocol's
expectation of idempotent operations.

cmd/serve/docker/volume.go

@@ -20,7 +20,6 @@ import (
 // Errors
 var (
 	ErrVolumeNotFound   = errors.New("volume not found")
-	ErrVolumeExists     = errors.New("volume already exists")
 	ErrMountpointExists = errors.New("non-empty mountpoint already exists")
 )