dockerfile: avoid frontend panic when no stages defined

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>

frontend/dockerfile/dockerfile2llb/convert.go

@@ -258,6 +258,9 @@ func toDispatchState(ctx context.Context, dt []byte, opt ConvertOpt) (*dispatchS
 	if err != nil {
 		return nil, err
 	}
+	if len(stages) == 0 {
+		return nil, errors.New("dockerfile contains no stages to build")
+	}
 	validateStageNames(stages, lint)
 
 	shlex := shell.NewLex(dockerfile.EscapeToken)

frontend/dockerfile/dockerfile_test.go

@@ -196,6 +196,7 @@ var allTests = integration.TestFuncs(
 	testInvalidJSONCommands,
 	testHistoryError,
 	testHistoryFinalizeTrace,
+	testEmptyStages,
 )
 
 // Tests that depend on the `security.*` entitlements
@@ -5479,6 +5480,40 @@ RUN echo $(hostname) | grep foo
 	}
 }
 
+func testEmptyStages(t *testing.T, sb integration.Sandbox) {
+	integration.SkipOnPlatform(t, "windows")
+	f := getFrontend(t, sb)
+	dockerfile := []byte(`
+ARG foo=bar
+`)
+
+	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{
+		LocalMounts: map[string]fsutil.FS{
+			dockerui.DefaultLocalNameDockerfile: dir,
+			dockerui.DefaultLocalNameContext:    dir,
+		},
+		Exports: []client.ExportEntry{
+			{
+				Type:      client.ExporterLocal,
+				OutputDir: destDir,
+			},
+		},
+	}, nil)
+	require.Error(t, err)
+	require.Contains(t, err.Error(), "dockerfile contains no stages to build")
+}
+
 func testShmSize(t *testing.T, sb integration.Sandbox) {
 	integration.SkipOnPlatform(t, "windows")
 	f := getFrontend(t, sb)