Properly mount base layers

As opposed to a writable layer derived from a base layer, the volume
path of a base layer, once activated and prepared will not be a WCIFS
volume, but the actual path on disk to the snapshot. We cannot directly
mount this folder, as that would mean a client may gain access and
potentially damage important metadata files that would render the layer
unusabble.

For base layers we need to mount the Files folder which must exist in
any valid base windows-layer.

Signed-off-by: Gabriel Adrian Samfira <gsamfira@cloudbasesolutions.com>

snapshots/windows/windows.go

@@ -319,16 +319,6 @@ func (s *snapshotter) mounts(sn storage.Snapshot, key string) []mount.Mount {
 
 	mountType := "windows-layer"
 
-	if len(sn.ParentIDs) == 0 {
-		// A mount of a parentless snapshot is a bind-mount.
-		mountType = "bind"
-		// If not being extracted into, then the bind-target is the
-		// "Files" subdirectory.
-		if !strings.Contains(key, snapshots.UnpackKeyPrefix) {
-			source = filepath.Join(source, "Files")
-		}
-	}
-
 	// error is not checked here, as a string array will never fail to Marshal
 	parentLayersJSON, _ := json.Marshal(parentLayerPaths)
 	parentLayersOption := mount.ParentLayerPathsFlag + string(parentLayersJSON)
@@ -336,7 +326,7 @@ func (s *snapshotter) mounts(sn storage.Snapshot, key string) []mount.Mount {
 	options := []string{
 		roFlag,
 	}
-	if mountType != "bind" {
+	if len(sn.ParentIDs) != 0 {
 		options = append(options, parentLayersOption)
 	}
 	mounts := []mount.Mount{