Make portability tests pull Docker images from Dockerhub

tools/internal_ci/linux/grpc_portability_build_only.sh

@@ -35,4 +35,7 @@ cd $(dirname $0)/../../..
 
 git submodule update --init
 
+# download docker images from dockerhub
+export DOCKERHUB_ORGANIZATION=grpctesting
+
 tools/jenkins/run_jenkins_matrix.sh -f portability linux --build_only