pkg/broadcastwriter: avoid alloc w/ WriteString

Signed-off-by: Cristian Staretu <cristian.staretu@gmail.com>
(cherry picked from commit db877d8a42)

Docker-DCO-1.1-Signed-off-by: Jessie Frazelle <hugs@docker.com> (github: jfrazelle)

Docker-DCO-1.1-Signed-off-by: Jessie Frazelle <jess@docker.com> (github: jfrazelle)

pkg/broadcastwriter/broadcastwriter.go

@@ -51,7 +51,7 @@ func (w *BroadcastWriter) Write(p []byte) (n int, err error) {
 	for {
 		line, err := w.buf.ReadString('\n')
 		if err != nil {
-			w.buf.Write([]byte(line))
+			w.buf.WriteString(line)
 			break
 		}
 		for stream, writers := range w.streams {