add keep-cache option to preserve intermediate docker images

Signed-off-by: Adphi <philippe.adrien.nousse@gmail.com>

convert_options.go

@@ -28,6 +28,8 @@ type convertOptions struct {
 	bootSize  uint64
 
 	luksPassword string
+
+	keepCache bool
 }
 
 func WithSize(size uint64) ConvertOption {
@@ -83,3 +85,9 @@ func WithLuksPassword(password string) ConvertOption {
 		o.luksPassword = password
 	}
 }
+
+func WithKeepCache(b bool) ConvertOption {
+	return func(o *convertOptions) {
+		o.keepCache = b
+	}
+}