osbuild: add Transform to TarStageOptions

This commit adds the new `transform` option to the tar stage. This
allows chaning filenames while runnig tar and will be used by the
GCE pipeline for bootc-disk.

See also osbuild/osbuild#1886

pkg/osbuild/tar_stage.go

@@ -42,6 +42,9 @@ type TarStageOptions struct {
 
 	// List of paths to include, instead of the whole tree
 	Paths []string `json:"paths,omitempty"`
+
+	// Pass --transform=...
+	Transform string `json:"transform,omitempty"`
 }
 
 func (TarStageOptions) isStageOptions() {}

pkg/osbuild/tar_stage_test.go

@@ -1,6 +1,7 @@
 package osbuild
 
 import (
+	"encoding/json"
 	"testing"
 
 	"github.com/stretchr/testify/assert"
@@ -77,3 +78,16 @@ func TestTarStageOptionsValidate(t *testing.T) {
 		})
 	}
 }
+
+func TestTarStageOptionsJSON(t *testing.T) {
+	stageOptions := &TarStageOptions{
+		Filename:  "archive.tar.xz",
+		Transform: "s/foo/bar/",
+	}
+	b, err := json.MarshalIndent(stageOptions, "", "  ")
+	assert.NoError(t, err)
+	assert.Equal(t, string(b), `{
+  "filename": "archive.tar.xz",
+  "transform": "s/foo/bar/"
+}`)
+}