fix date command

.gitlab-ci.yml

 variables:
   IMAGE: rwthos/hermit-cargo
   GIT_SUBMODULE_STRATEGY: normal
-  VERSION: $(date %Y%m%d%H%M%S)
 
 stages:
   - prepare
@@ -12,6 +11,7 @@ stages:
 docker:
   stage: prepare
   script:
+    - export VERSION=$(date --rfc-3339='date')
     - docker build -t ${IMAGE} .
     - docker tag ${IMAGE} ${IMAGE}:latest
     - docker tag ${IMAGE} ${IMAGE}:${VERSION}