CentOS 8 is deprecated for core Dockerfile

[twtaylor]

If you try to stand-up the Dockerfile in core, you get:

> DOCKER_BUILDKIT=0 COMPOSE_DOCKER_CLI_BUILD=0  docker build -t subzero . --no-cache
Sending build context to Docker daemon  12.48MB
Step 1/14 : FROM centos:8.1.1911
8.1.1911: Pulling from library/centos
d6d1431672e7: Pull complete
Digest: sha256:fe8d824220415eed5477b63addf40fb06c3b049404242b31982106ac204f6700
Status: Downloaded newer image for centos:8.1.1911
 ---> 796035f59a88
Step 2/14 : RUN yum -y --enablerepo=extras install     autoconf     automake     clang     cmake     dnf-plugins-core     gcc     gcc-c++     git     libtool     make     python2     unzip     vim
 ---> Running in 1091d397e747
CentOS-8 - AppStream                            7.2  B/s |  38  B     00:05
Failed to download metadata for repo 'AppStream'
Error: Failed to download metadata for repo 'AppStream'
The command '/bin/sh -c yum -y --enablerepo=extras install     autoconf     automake     clang     cmake     dnf-plugins-core     gcc     gcc-c++     git     libtool     make     python2     unzip     vim' returned a non-zero code: 1

This is due to this tag being fairly outdated: https://hub.docker.com/layers/library/centos/centos8.1.1911/images/sha256-567785922b920b35aee6a217f70433fd437b335ad45054743c960d1aaa143dcd?context=explore

But also due to the fact the yum repository is no longer supported by CentOS