support using multiple snapshotters simultaneously

e.g. dist pull --snapshotter btrfs ...; ctr run --snapshotter btrfs ...
(empty string defaults for overlayfs)

Signed-off-by: Akihiro Suda <suda.akihiro@lab.ntt.co.jp>
Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

containers/containers.go

@@ -12,14 +12,15 @@ import (
 //
 // The resources specified in this object are used to create tasks from the container.
 type Container struct {
-	ID        string
-	Labels    map[string]string
-	Image     string
-	Runtime   RuntimeInfo
-	Spec      *types.Any
-	RootFS    string
-	CreatedAt time.Time
-	UpdatedAt time.Time
+	ID          string
+	Labels      map[string]string
+	Image       string
+	Runtime     RuntimeInfo
+	Spec        *types.Any
+	RootFS      string
+	Snapshotter string
+	CreatedAt   time.Time
+	UpdatedAt   time.Time
 }
 
 type RuntimeInfo struct {