test: set a name for CI Docker containers

ci/test/00_setup_env_s390x.sh

@@ -17,6 +17,7 @@ if [ -n "$QEMU_USER_CMD" ]; then
   export PACKAGES="$PACKAGES g++-s390x-linux-gnu qemu-user libc6:s390x libstdc++6:s390x libfontconfig1:s390x libxcb1:s390x"
 fi
 # Use debian to avoid 404 apt errors
+export CONTAINER_NAME=ci_s390x
 export DOCKER_NAME_TAG="debian:buster"
 export RUN_UNIT_TESTS=true
 export RUN_FUNCTIONAL_TESTS=true