feat: allow setting platform for scratch image

frontend/dockerfile/dockerfile2llb/convert.go

@@ -468,9 +468,11 @@ func toDispatchState(ctx context.Context, dt []byte, opt ConvertOpt) (*dispatchS
 			if d.base == nil && !d.dispatched && !d.resolved {
 				d.resolved = reachable // avoid re-resolving if called again after onbuild
 				if d.stage.BaseName == emptyImageName {
-					d.state = llb.Scratch()
-					d.image = emptyImage(platformOpt.targetPlatform)
-					d.platform = &platformOpt.targetPlatform
+					if d.platform == nil {
+						d.platform = &platformOpt.targetPlatform
+					}
+					d.state = llb.Scratch().Platform(*d.platform)
+					d.image = emptyImage(*d.platform)
 					if d.unregistered {
 						d.dispatched = true
 					}
@@ -511,7 +513,7 @@ func toDispatchState(ctx context.Context, dt []byte, opt ConvertOpt) (*dispatchS
 							if img != nil {
 								d.image = *img
 							} else {
-								d.image = emptyImage(platformOpt.targetPlatform)
+								d.image = emptyImage(*platform)
 							}
 							d.state = st.Platform(*platform)
 							d.platform = platform

frontend/dockerfile/dockerfile2llb/convert_test.go

@@ -9,6 +9,7 @@ import (
 	"github.com/moby/buildkit/frontend/dockerui"
 	"github.com/moby/buildkit/util/appcontext"
 	digest "github.com/opencontainers/go-digest"
+	ocispecs "github.com/opencontainers/image-spec/specs-go/v1"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 )
@@ -224,9 +225,43 @@ RUN echo foo
 FROM foo AS bar
 RUN echo bar
 `
-	_, _, baseImg, _, err := Dockerfile2LLB(appcontext.Context(), []byte(df), ConvertOpt{})
+	state, _, baseImg, _, err := Dockerfile2LLB(appcontext.Context(), []byte(df), ConvertOpt{TargetPlatform: &ocispecs.Platform{OS: "targetos", Architecture: "targetarch"}})
 	require.NoError(t, err)
+
+	platform, err := state.GetPlatform(context.TODO())
+	require.NoError(t, err)
+	assert.Equal(t, "linux", platform.OS)
+	assert.Equal(t, "amd64", platform.Architecture)
+
 	t.Logf("baseImg=%+v", baseImg)
+	assert.Equal(t, "linux", baseImg.Platform.OS)
+	assert.Equal(t, "amd64", baseImg.Platform.Architecture)
 	assert.Equal(t, []digest.Digest{"sha256:2e112031b4b923a873c8b3d685d48037e4d5ccd967b658743d93a6e56c3064b9"}, baseImg.RootFS.DiffIDs)
 	assert.Equal(t, "2024-01-17 21:49:12 +0000 UTC", baseImg.Created.String())
 }
+
+func TestBaseImageScratchPlatform(t *testing.T) {
+	// Defaults to targetplatform
+	df := `FROM scratch`
+	state, _, baseImg, _, err := Dockerfile2LLB(appcontext.Context(), []byte(df), ConvertOpt{TargetPlatform: &ocispecs.Platform{OS: "targetos", Architecture: "targetarch"}})
+	require.NoError(t, err)
+
+	platform, err := state.GetPlatform(context.TODO())
+	require.NoError(t, err)
+	assert.Equal(t, "targetos", platform.OS)
+	assert.Equal(t, "targetarch", platform.Architecture)
+
+	assert.Nil(t, baseImg)
+
+	// Cross-compilation scenario
+	df = `FROM --platform=linux/ppc64le scratch`
+	state, _, baseImg, _, err = Dockerfile2LLB(appcontext.Context(), []byte(df), ConvertOpt{TargetPlatform: &ocispecs.Platform{OS: "targetos", Architecture: "targetarch"}})
+	require.NoError(t, err)
+
+	platform, err = state.GetPlatform(context.TODO())
+	require.NoError(t, err)
+	assert.Equal(t, "linux", platform.OS)
+	assert.Equal(t, "ppc64le", platform.Architecture)
+
+	assert.Nil(t, baseImg)
+}

frontend/dockerfile/dockerfile_test.go

@@ -109,6 +109,7 @@ var allTests = integration.TestFuncs(
 	testHTTPDockerfile,
 	testPlatformArgsImplicit,
 	testPlatformArgsExplicit,
+	testPlatformArgsExplicitContext,
 	testExportMultiPlatform,
 	testQuotedMetaArgs,
 	testGlobalArgErrors,
@@ -6310,6 +6311,61 @@ COPY --from=build out .
 	require.Equal(t, "freebsd", string(dt))
 }
 
+func testPlatformArgsExplicitContext(t *testing.T, sb integration.Sandbox) {
+	integration.SkipOnPlatform(t, "windows")
+	f := getFrontend(t, sb)
+
+	dockerfile := []byte(`
+FROM --platform=$BUILDPLATFORM scratch AS build
+COPY --from=bb . /
+ARG TARGETPLATFORM
+ARG TARGETOS
+RUN mkdir /out && echo -n $TARGETPLATFORM > /out/platform && echo -n $TARGETOS > /out/os
+FROM scratch
+COPY --from=build 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()
+
+	opt := client.SolveOpt{
+		Exports: []client.ExportEntry{
+			{
+				Type:      client.ExporterLocal,
+				OutputDir: destDir,
+			},
+		},
+		FrontendAttrs: map[string]string{
+			"platform":           "darwin/ppc64le",
+			"build-arg:TARGETOS": "freebsd",
+			"context:bb":         "docker-image://busybox",
+		},
+		LocalMounts: map[string]fsutil.FS{
+			dockerui.DefaultLocalNameDockerfile: dir,
+			dockerui.DefaultLocalNameContext:    dir,
+		},
+	}
+
+	_, err = f.Solve(sb.Context(), c, opt, nil)
+	require.NoError(t, err)
+
+	dt, err := os.ReadFile(filepath.Join(destDir, "platform"))
+	require.NoError(t, err)
+	require.Equal(t, "darwin/ppc64le", string(dt))
+
+	dt, err = os.ReadFile(filepath.Join(destDir, "os"))
+	require.NoError(t, err)
+	require.Equal(t, "freebsd", string(dt))
+}
+
 func testBuiltinArgs(t *testing.T, sb integration.Sandbox) {
 	f := getFrontend(t, sb)