Centos8: move to stream, add basetag instead of latest for this platform
This change lets one set the tag used for the centos8 (stream) docker image, either directly in the gitlab-ci.yml file or via variables set for schedules or runs.
This change lets one set the tag used for the centos8 (stream) docker image, either directly in the gitlab-ci.yml file or via variables set for schedules or runs.