Use proper way for passing env var as artifact

.gitlab-ci.yml

@@ -58,6 +58,8 @@ build ci:
     # most recent docker image should be tagged with most recent SHA
     - docker_tag=$(git rev-parse --short HEAD)
     - echo $docker_tag
+    # make the docker tag available in later stages
+    - echo export docker_tag=$docker_tag >cibuild.env
     - cd chart
     - |-
       sed -i "s/^appVersion: .*\$/appVersion: ${docker_tag}/g" ${SUBCOMP_CHART_DIR}/Chart.yaml
@@ -66,8 +68,7 @@ build ci:
   artifacts:
     paths:
       - chart/${SUBCOMP_CHART_DIR}
-    variables:
-      - docker_tag
+      - cibuild.env
     expire_in: 6 hours
 # XXX: Do we always want to clone the subcomponent
 #  rules: