Update klarlack to version 20210529.

This is due at any rate, and in particular the more recent versions
of VMOD re2 and selector support the VCL_SUB feature.

container/Dockerfile.klarlack

 FROM ubuntu:bionic-20201119
-ENV VERSION=20210409+bionic
+ENV VERSION=20210529+bionic
 
 RUN apt-get update && \
 	apt-get upgrade -y && \