diff --git a/cli/command/image/build_session.go b/cli/command/image/build_session.go
index 897d237cd6..f33580bd70 100644
--- a/cli/command/image/build_session.go
+++ b/cli/command/image/build_session.go
@@ -122,6 +122,10 @@ func (bw *bufferedWriter) flushBuffer() {
 	bw.mu.Unlock()
 }
 
+func (bw *bufferedWriter) String() string {
+	return fmt.Sprintf("%s", bw.Writer)
+}
+
 func getBuildSharedKey(dir string) (string, error) {
 	// build session is hash of build dir with node based randomness
 	s := sha256.Sum256([]byte(fmt.Sprintf("%s:%s", tryNodeIdentifier(), dir)))