Wait for stdio to close

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>

containerd-shim/main.go

@@ -117,6 +117,7 @@ func main() {
 		}
 		// runtime has exited so the shim can also exit
 		if exitShim {
+			p.Wait()
 			return
 		}
 	}