parser: modify tips when no instruction found in Dockerfile

Signed-off-by: Nliuzekun <liuzekun@huawei.com>

builder/dockerfile/parser/parser.go

@@ -184,7 +184,7 @@ func getPageName(line *parser.Line, pageNum int) (string, error) {
 
 func constructPages(lines []*parser.Line, onbuild bool) ([]*parser.Page, error) {
 	if len(lines) == 0 {
-		return nil, errors.New("dockerfile content is empty")
+		return nil, errors.New("no instructions in Dockerfile")
 	}
 
 	var (

builder/dockerfile/parser/parser_test.go

@@ -142,7 +142,7 @@ func TestParse(t *testing.T) {
 		{
 			name:   "busybox_with_empty_content",
 			isErr:  true,
-			errStr: "dockerfile content is empty",
+			errStr: "no instructions in Dockerfile",
 		},
 		{
 			name:   "busybox_no_command",

daemon/build.go

@@ -68,7 +68,7 @@ func (b *Backend) Build(req *pb.BuildRequest, stream pb.Control_BuildServer) (er
 		// the pipeFile, which will cause frontend hangs forever.
 		// so if the output type is archive(pipeFile is not empty string) and any error occurred, we write the error
 		// message into the pipe to make the goroutine move on instead of hangs.
-		if err != nil && pipeWrapper.PipeFile != "" {
+		if err != nil && pipeWrapper != nil {
 			pipeWrapper.Close()
 			if perr := ioutil.WriteFile(pipeWrapper.PipeFile, []byte(err.Error()), constant.DefaultRootFileMode); perr != nil {
 				logrus.WithField(util.LogKeySessionID, req.BuildID).Warnf("Write error [%v] in to pipe file failed: %v", err, perr)