Merge pull request moby#3130 from jedevc/named-image-context-scratch

dockerfile: allow named context for docker-image://scratch

Author: tonistiigi

frontend/dockerfile/builder/build.go

@@ -848,6 +848,11 @@ func contextByName(ctx context.Context, c client.Client, sessionID, name string,
 	switch vv[0] {
 	case "docker-image":
 		ref := strings.TrimPrefix(vv[1], "//")
+		if ref == "scratch" {
+			st := llb.Scratch()
+			return &st, nil, nil, nil
+		}
+
 		imgOpt := []llb.ImageOption{
 			llb.WithCustomName("[context " + name + "] " + ref),
 		}

frontend/dockerfile/dockerfile_test.go

@@ -117,6 +117,7 @@ var allTests = integration.TestFuncs(
 	testNamedImageContext,
 	testNamedImageContextPlatform,
 	testNamedImageContextTimestamps,
+	testNamedImageContextScratch,
 	testNamedLocalContext,
 	testNamedOCILayoutContext,
 	testNamedOCILayoutContextExport,
@@ -5350,6 +5351,57 @@ RUN echo foo >> /test
 	require.Less(t, diff, 10*time.Minute)
 }
 
+func testNamedImageContextScratch(t *testing.T, sb integration.Sandbox) {
+	ctx := sb.Context()
+
+	c, err := client.New(ctx, sb.Address())
+	require.NoError(t, err)
+	defer c.Close()
+
+	dockerfile := []byte(`
+FROM busybox
+COPY <<EOF /out
+hello world!
+EOF
+`)
+
+	dir, err := integration.Tmpdir(
+		t,
+		fstest.CreateFile("Dockerfile", dockerfile, 0600),
+	)
+	require.NoError(t, err)
+
+	f := getFrontend(t, sb)
+
+	destDir := t.TempDir()
+
+	_, err = f.Solve(sb.Context(), c, client.SolveOpt{
+		FrontendAttrs: map[string]string{
+			"context:busybox": "docker-image://scratch",
+		},
+		LocalDirs: map[string]string{
+			builder.DefaultLocalNameDockerfile: dir,
+			builder.DefaultLocalNameContext:    dir,
+		},
+		Exports: []client.ExportEntry{
+			{
+				Type:      client.ExporterLocal,
+				OutputDir: destDir,
+			},
+		},
+	}, nil)
+	require.NoError(t, err)
+
+	items, err := os.ReadDir(destDir)
+	require.NoError(t, err)
+	require.Equal(t, 1, len(items))
+	require.Equal(t, "out", items[0].Name())
+
+	dt, err := os.ReadFile(filepath.Join(destDir, "out"))
+	require.NoError(t, err)
+	require.Equal(t, "hello world!\n", string(dt))
+}
+
 func testNamedLocalContext(t *testing.T, sb integration.Sandbox) {
 	ctx := sb.Context()