service/snapshotter: move default to client

In order to enforce strict handling of snapshotter values on the
container object, the defaults have been moved to the client side. This
ensures that we correctly qualify the snapshotter under use when from
the container at the time it was created, rather than possibly losing
the metadata on a change of default.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

container.go

@@ -174,6 +174,10 @@ func (c *container) NewTask(ctx context.Context, ioCreate IOCreation, opts ...Ne
 		Stderr:      cfg.Stderr,
 	}
 	if c.c.RootFS != "" {
+		if c.c.Snapshotter == "" {
+			return nil, errors.Wrapf(errdefs.ErrInvalidArgument, "unable to resolve rootfs mounts without snapshotter on container")
+		}
+
 		// get the rootfs from the snapshotter and add it to the request
 		mounts, err := c.client.SnapshotService(c.c.Snapshotter).Mounts(ctx, c.c.RootFS)
 		if err != nil {