Update to CERN CentOS 7.9 and use CERN GitLab registry

cc7-cms/Dockerfile

 # CC7 OS capable of using/running CMS software release(s).
 
 # Make the base image configurable:
-ARG BASEIMAGE=cern/cc7-base:20200901-1.x86_64
+ARG BASEIMAGE=gitlab-registry.cern.ch/linuxsupport/cc7-base:20201116-1.x86_64
 
 # Set up the CC7 CMSSW base:
 FROM ${BASEIMAGE}