Revert ".gitlab-ci.yml: remove nightly-$(date) again"

This reverts commit 50ac95f5.

.gitlab-ci.yml

@@ -312,6 +312,11 @@ jug_xl:nightly:
     - ./gitlab-ci/docker_push.sh -i ${BUILD_IMAGE} -l ${NIGHTLY_TAG} 
                                  -n $DOCKER_NTRIES -t $DOCKER_WAIT_TIME
                                  ${NIGHTLY_TAG} ${PUSH_FLAG}
+    - if [ -z "${PUSH_FLAG}" ] ; then 
+      ./gitlab-ci/docker_push.sh -i ${BUILD_IMAGE} -l ${NIGHTLY_TAG} 
+                                 -n $DOCKER_NTRIES -t $DOCKER_WAIT_TIME
+                                 ${NIGHTLY_TAG}-$(date +%Y-%m-%d) --dockerhub ;
+      fi
 
 jug_xl:feature:
   extends: .build