Embedded containerd always derives its persistent root from Docker's
data root, which prevents testing against state from a standalone
containerd daemon.
Use DOCKER_CONTAINERD_ROOT as a temporary override for the exact root
path used by embedded and supervised containerd. Preserve the existing
data-root/containerd/daemon default when the variable is unset.
Signed-off-by: Paweł Gronowski <pawel.gronowski@docker.com>