Update CC7 base image to 20200531-2.x86_64

cc7-cms/Dockerfile

 # CC7 OS capable of using/running CMS software release(s).
 
 # Make the base image configurable:
-ARG BASEIMAGE=cern/cc7-base:20191107
+ARG BASEIMAGE=cern/cc7-base:20200531-2.x86_64
 
 # Set up the CC7 CMSSW base:
 FROM ${BASEIMAGE}