Merge pull request #1633 from wangyumu/add-buildkit-syntax-build-arg

build-arg add support BUILDKIT_SYNTAX

frontend/dockerfile/builder/build.go

@@ -53,6 +53,7 @@ const (
 	keyNameDockerfile          = "dockerfilekey"
 	keyContextSubDir           = "contextsubdir"
 	keyContextKeepGitDir       = "build-arg:BUILDKIT_CONTEXT_KEEP_GIT_DIR"
+	keySyntax                  = "build-arg:BUILDKIT_SYNTAX"
 )
 
 var httpPrefix = regexp.MustCompile(`^https?://`)
@@ -317,8 +318,14 @@ func Build(ctx context.Context, c client.Client) (*client.Result, error) {
 	}
 
 	if _, ok := opts["cmdline"]; !ok {
-		ref, cmdline, loc, ok := dockerfile2llb.DetectSyntax(bytes.NewBuffer(dtDockerfile))
-		if ok {
+		if cmdline, ok := opts[keySyntax]; ok {
+			p := strings.SplitN(strings.TrimSpace(cmdline), " ", 2)
+			res, err := forwardGateway(ctx, c, p[0], cmdline)
+			if err != nil && len(errdefs.Sources(err)) == 0 {
+				return nil, errors.Wrapf(err, "failed with %s = %s", keySyntax, cmdline)
+			}
+			return res, err
+		} else if ref, cmdline, loc, ok := dockerfile2llb.DetectSyntax(bytes.NewBuffer(dtDockerfile)); ok {
 			res, err := forwardGateway(ctx, c, ref, cmdline)
 			if err != nil && len(errdefs.Sources(err)) == 0 {
 				return nil, wrapSource(err, sourceMap, loc)