examples/dockerfile2llb: add --image-json-file, --metadata-file

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

examples/dockerfile2llb/main.go

@@ -2,6 +2,7 @@ package main
 
 import (
 	"context"
+	"encoding/json"
 	"flag"
 	"io"
 	"os"
@@ -15,7 +16,9 @@ import (
 )
 
 type buildOpt struct {
-	target string
+	target        string
+	imageJSONFile string
+	metadataFile  string
 }
 
 func main() {
@@ -27,6 +30,8 @@ func main() {
 func xmain() error {
 	var opt buildOpt
 	flag.StringVar(&opt.target, "target", "", "target stage")
+	flag.StringVar(&opt.imageJSONFile, "image-json-file", "", "Output image config to a file as JSON")
+	flag.StringVar(&opt.metadataFile, "metadata-file", "", "Output build metadata (e.g., image digest) to a file as JSON")
 	flag.Parse()
 
 	df, err := io.ReadAll(os.Stdin)
@@ -45,12 +50,33 @@ func xmain() error {
 		return err
 	}
 
-	_ = img
-	_ = bi
-
 	dt, err := state.Marshal(context.TODO())
 	if err != nil {
 		return err
 	}
-	return llb.WriteTo(dt, os.Stdout)
+	if err := llb.WriteTo(dt, os.Stdout); err != nil {
+		return err
+	}
+	if opt.imageJSONFile != "" {
+		if err := writeJSON(opt.imageJSONFile, img); err != nil {
+			return err
+		}
+	}
+	if opt.metadataFile != "" {
+		if err := writeJSON(opt.metadataFile, bi); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+func writeJSON(f string, x interface{}) error {
+	b, err := json.Marshal(x)
+	if err != nil {
+		return err
+	}
+	if err := os.RemoveAll(f); err != nil {
+		return err
+	}
+	return os.WriteFile(f, b, 0o644)
 }