helper/bwrap: implement overlayfs builder

Signed-off-by: Ophestra <cat@gensokyo.uk>

helper/bwrap/arg.static.awkward.go

@@ -4,10 +4,20 @@ const (
 	Tmpfs = iota
 	Dir
 	Symlink
+
+	OverlaySrc
+	Overlay
+	TmpOverlay
+	ROOverlay
 )
 
 var awkwardArgs = [...]string{
 	Tmpfs:   "--tmpfs",
 	Dir:     "--dir",
 	Symlink: "--symlink",
+
+	OverlaySrc: "--overlay-src",
+	Overlay:    "--overlay",
+	TmpOverlay: "--tmp-overlay",
+	ROOverlay:  "--ro-overlay",
 }