CI: use proper ref name to maintain dots in version numbers

.gitlab-ci.yml

@@ -10,7 +10,7 @@ docker:
     entrypoint: [""]
   script:
     - |
-        CONTAINER_TAG="${CI_COMMIT_REF_SLUG}"
+        CONTAINER_TAG="${CI_COMMIT_REF_NAME}"
         if [ "${CONTAINER_TAG}" = "${CI_DEFAULT_BRANCH}" ]; then
           CONTAINER_TAG="latest"
         fi