Switch CI from CentOS8 to Almalinux

The open_pdks CI is no longer working because it relies on CentOS8
which has been EOL for over a year.

.github/Dockerfile

diff --git a/.github/Dockerfile b/.github/Dockerfile
index 44334c7..33b56e4 100644
--- a/.github/Dockerfile
+++ b/.github/Dockerfile
@@ -13,7 +13,7 @@
 # limitations under the License.
 
 # syntax = docker/dockerfile:1.0-experimental
-FROM centos:centos8 as build
+FROM almalinux:8 as build
 
 # Common development tools and libraries (kitchen sink approach)
 RUN yum groupinstall -y "Development Tools"