switch to using almalinux base-8 image for rpmbuild

due to centos8 EOL we are not able to use that image
for building rpms on the CI

We are sticking with an el8 distro because our release
builds will be made in a rhel8 build root, and thus we
need to check this does not break.

Containerfile.rpmbuild

@@ -1,4 +1,4 @@
-FROM quay.io/centos/centos:stream8
+FROM quay.io/almalinuxorg/8-base:8
 WORKDIR $APP_ROOT/src
 RUN yum -y install git-core rpm-build dnf-plugins-core 'dnf-command(builddep)'
 COPY . .