ppc64le: Switch Build Containers Job to Buildah
ppc64le: Switch Build Containers Job to Buildah

This patch sets a variable so the build containers job will use
buildah rather than docker, as docker is no longer supported with
centos 8.

Change-Id: Iead11deeed2f45733d36272eabe5d091d2005ca5
Co-Authored-By: Mike Turek <4b55cb3828cd7561bbc232dfef85cae8beda75f3@linux.vnet.ibm.com>

rdo-infra/ci-config,redhat-openstack/rdo-infra,redhat-openstack/rdo-infra,rdo-infra/ci-config,rdo-infra/ci-config,redhat-openstack/rdo-infra,rdo-infra/ci-config,redhat-openstack/rdo-infra