imagetools inspect: use buildinfo helper

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

vendor/github.com/moby/buildkit/util/imageutil/buildinfo.go

@@ -0,0 +1,32 @@
+package imageutil
+
+import (
+	"encoding/base64"
+	"encoding/json"
+
+	binfotypes "github.com/moby/buildkit/util/buildinfo/types"
+	"github.com/pkg/errors"
+)
+
+// BuildInfo returns build info from image config.
+func BuildInfo(dt []byte) (*binfotypes.BuildInfo, error) {
+	if len(dt) == 0 {
+		return nil, nil
+	}
+	var config binfotypes.ImageConfig
+	if err := json.Unmarshal(dt, &config); err != nil {
+		return nil, errors.Wrap(err, "failed to unmarshal image config")
+	}
+	if len(config.BuildInfo) == 0 {
+		return nil, nil
+	}
+	dtbi, err := base64.StdEncoding.DecodeString(config.BuildInfo)
+	if err != nil {
+		return nil, err
+	}
+	var bi binfotypes.BuildInfo
+	if err = json.Unmarshal(dtbi, &bi); err != nil {
+		return nil, errors.Wrap(err, "failed to decode buildinfo from image config")
+	}
+	return &bi, nil
+}