info: add driver-type

With this patch:

    mkdir -p /etc/docker/
    echo '{"features":{"containerd-snapshotter":true}}' > /etc/docker/daemon.json
    dockerd

    docker info
    ...
    Storage Driver: overlayfs
     driver-type: io.containerd.snapshotter.v1
    Logging Driver: json-file

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

daemon/containerd/service.go

@@ -4,6 +4,7 @@ import (
 	"context"
 
 	"github.com/containerd/containerd"
+	"github.com/containerd/containerd/plugin"
 	"github.com/docker/docker/api/types"
 	"github.com/docker/docker/container"
 	"github.com/docker/docker/daemon/images"
@@ -64,7 +65,10 @@ func (i *ImageService) GetLayerByID(cid string) (layer.RWLayer, error) {
 // LayerStoreStatus returns the status for each layer store
 // called from info.go
 func (i *ImageService) LayerStoreStatus() [][2]string {
-	return [][2]string{}
+	// TODO(thaJeztah) do we want to add more details about the driver here?
+	return [][2]string{
+		{"driver-type", string(plugin.SnapshotPlugin)},
+	}
 }
 
 // GetLayerMountID returns the mount ID for a layer