Update CI docker images

espressofoundation/ubuntu:latest was 3 years old and unmaintained. Indeed, it comes with buggy GCC 7.3. It is better to use a well maintained official ubuntu 18.04 image with GCC 7.5 now.

Also replaced

nvcr.io/nvidia/nvhpc:20.9-devel-centos7

with

nvcr.io/nvidia/nvhpc:20.9-devel-ubuntu20.04

to avoid dealing with aging centos7.

Then all the CI images are ubuntu based.