Avoid v1.Manifest in crane edit config

Because of how omitempty works, an empty Layers and a nil Layers will
both be omitted. We don't want that, because different registries
support different variants of that. Instead, we just pass through
whatever was there before by treating the manifest as a map[string]any
and manipulating that rather than using v1.Manifest-based helpers.

internal/cmd/edit.go

@@ -161,15 +161,37 @@ func editConfig(in io.Reader, out io.Writer, src, dst string, options ...crane.O
 		return nil, err
 	}
 
-	m, err := img.Manifest()
+	mt, err := img.MediaType()
 	if err != nil {
 		return nil, err
 	}
-	mt, err := img.MediaType()
+
+	// We want to omit Layers in certain situations, so we don't use v1.Image.Manifest() here.
+	// Instead, we treat the manifest as a map[string]any and just manipulate the config desc.
+	mb, err := img.RawManifest()
 	if err != nil {
 		return nil, err
 	}
 
+	jsonMap := map[string]any{}
+	if err := json.Unmarshal(mb, &jsonMap); err != nil {
+		return nil, err
+	}
+
+	cv, ok := jsonMap["config"]
+	if !ok {
+		return nil, fmt.Errorf("config missing")
+	}
+	cb, err := json.Marshal(cv)
+	if err != nil {
+		return nil, fmt.Errorf("json.Marshal config: %w", err)
+	}
+
+	config := v1.Descriptor{}
+	if err := json.Unmarshal(cb, &config); err != nil {
+		return nil, fmt.Errorf("json.Unmarshal config: %w", err)
+	}
+
 	var edited []byte
 	if interactive(in, out) {
 		rcf, err := img.RawConfigFile()
@@ -188,15 +210,17 @@ func editConfig(in io.Reader, out io.Writer, src, dst string, options ...crane.O
 		edited = b
 	}
 
-	l := static.NewLayer(edited, m.Config.MediaType)
+	l := static.NewLayer(edited, config.MediaType)
 	layerDigest, err := l.Digest()
 	if err != nil {
 		return nil, err
 	}
 
-	m.Config.Digest = layerDigest
-	m.Config.Size = int64(len(edited))
-	b, err := json.Marshal(m)
+	config.Digest = layerDigest
+	config.Size = int64(len(edited))
+
+	jsonMap["config"] = config
+	b, err := json.Marshal(jsonMap)
 	if err != nil {
 		return nil, err
 	}