dockerfile: ignore empty BUILDKIT_SYNTAX overrides

Signed-off-by: CrazyMax <1951866+crazy-max@users.noreply.github.com>

Author: crazy-max

frontend/dockerfile/builder/build.go

@@ -52,13 +52,15 @@ func Build(ctx context.Context, c client.Client) (_ *client.Result, err error) {
 	}
 
 	if _, ok := opts["cmdline"]; !ok {
-		if cmdline, ok := opts[keySyntaxArg]; 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", keySyntaxArg, cmdline)
+		if v, ok := opts[keySyntaxArg]; ok {
+			if cmdline := strings.TrimSpace(v); cmdline != "" {
+				p := strings.SplitN(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", keySyntaxArg, cmdline)
+				}
+				return res, err
 			}
-			return res, err
 		} else if ref, cmdline, loc, ok := parser.DetectSyntax(src.Data); ok {
 			res, err := forwardGateway(ctx, c, ref, cmdline)
 			if err != nil && len(errdefs.Sources(err)) == 0 {

frontend/dockerfile/dockerfile_test.go

@@ -108,6 +108,7 @@ var allTests = integration.TestFuncs(
 	testCacheMountModeNoCache,
 	testDockerfileFromHTTP,
 	testBuiltinArgs,
+	testBuiltinSyntaxOverrideEmpty,
 	testPullScratch,
 	testSymlinkDestination,
 	testHTTPDockerfile,
@@ -6886,6 +6887,61 @@ COPY --from=build out /
 	require.Equal(t, expectedStr, string(dt))
 }
 
+func testBuiltinSyntaxOverrideEmpty(t *testing.T, sb integration.Sandbox) {
+	f := getFrontend(t, sb)
+
+	dockerfile := []byte(integration.UnixOrWindows(
+		`
+# syntax=docker/dockerfile-upstream:master
+FROM busybox AS build
+RUN echo -n ok > /out
+FROM scratch
+COPY --from=build /out /out
+`,
+		`
+# syntax=docker/dockerfile-upstream:master
+FROM nanoserver:latest AS build
+USER ContainerAdministrator
+RUN echo ok>C:\out
+FROM nanoserver:latest
+USER ContainerAdministrator
+COPY --from=build /out /out
+`,
+	))
+
+	dir := integration.Tmpdir(
+		t,
+		fstest.CreateFile("Dockerfile", dockerfile, 0600),
+	)
+
+	c, err := client.New(sb.Context(), sb.Address())
+	require.NoError(t, err)
+	defer c.Close()
+
+	destDir := t.TempDir()
+
+	_, err = f.Solve(sb.Context(), c, client.SolveOpt{
+		FrontendAttrs: map[string]string{
+			"build-arg:BUILDKIT_SYNTAX": "",
+		},
+		Exports: []client.ExportEntry{
+			{
+				Type:      client.ExporterLocal,
+				OutputDir: destDir,
+			},
+		},
+		LocalMounts: map[string]fsutil.FS{
+			dockerui.DefaultLocalNameDockerfile: dir,
+			dockerui.DefaultLocalNameContext:    dir,
+		},
+	}, nil)
+	require.NoError(t, err)
+
+	dt, err := os.ReadFile(filepath.Join(destDir, "out"))
+	require.NoError(t, err)
+	require.Equal(t, "ok", strings.TrimSpace(string(dt)))
+}
+
 func testTarContext(t *testing.T, sb integration.Sandbox) {
 	f := getFrontend(t, sb)
 

frontend/dockerfile/docs/reference.md

@@ -2723,7 +2723,7 @@ RUN echo "I'm building for $TARGETPLATFORM"
 | `BUILDKIT_INLINE_CACHE`[^2]      | Bool   | Inline cache metadata to image config or not.                                                                                                                                                                    |
 | `BUILDKIT_MULTI_PLATFORM`        | Bool   | Opt into deterministic output regardless of multi-platform output or not.                                                                                                                                        |
 | `BUILDKIT_SANDBOX_HOSTNAME`      | String | Set the hostname (default `buildkitsandbox`)                                                                                                                                                                     |
-| `BUILDKIT_SYNTAX`                | String | Set frontend image                                                                                                                                                                                               |
+| `BUILDKIT_SYNTAX`                | String | Set frontend image. Set to an empty value to ignore the Dockerfile `# syntax=` directive and use the built-in frontend instead.                                                                                 |
 | `SOURCE_DATE_EPOCH`              | Int    | Set the Unix timestamp for created image and layers. More info from [reproducible builds](https://reproducible-builds.org/docs/source-date-epoch/). Supported since Dockerfile 1.5, BuildKit 0.11                |
 
 #### Example: keep `.git` dir