Add WithReadonlyTempMount to create readonly temporary mounts

This is necessary so we can mount snapshots more than once with overlayfs,
otherwise mounts enter an unknown state.

related: https://github.com/moby/buildkit/pull/1100

Signed-off-by: Laura Brehm <laurabrehm@hey.com>
Co-authored-by: Zou Nengren <zouyee1989@gmail.com>

mount/temp.go

@@ -67,6 +67,13 @@ func WithTempMount(ctx context.Context, mounts []Mount, f func(root string) erro
 	return nil
 }
 
+// WithReadonlyTempMount mounts the provided mounts to a temp dir as readonly,
+// and pass the temp dir to f. The mounts are valid during the call to the f.
+// Finally we will unmount and remove the temp dir regardless of the result of f.
+func WithReadonlyTempMount(ctx context.Context, mounts []Mount, f func(root string) error) (err error) {
+	return WithTempMount(ctx, readonlyMounts(mounts), f)
+}
+
 func getTempDir() string {
 	if xdg := os.Getenv("XDG_RUNTIME_DIR"); xdg != "" {
 		return xdg