[ci] add automatic calendar version tag for jupyterhub image

.gitlab-ci.yml

@@ -20,6 +20,9 @@ deploy ubuntu:
   script:
     - image_name="${CI_REGISTRY}/${CI_PROJECT_PATH,,}"
     - podman push "${image_name}/ubuntu"
-    - podman push "${image_name}/jupyterhub"
+    - calver="$(date +'%Y-%m-%dT%H:%M')"
+    - podman tag "${image_name}/jupyterhub" "${image_name}/jupyterhub:$calver"
+    - podman push "${image_name}/jupyterhub:$calver"
+    - podman image rm "${image_name}/jupyterhub:$calver"
   after_script:
     - podman logout "${CI_REGISTRY}"