add env var for registry

.gitlab-ci.yml

 # In case something goes horribly wrong, you can fall back to `image: ruby:latest`
-image: git-r3lab.uni.lu:4567/r3/docker/jekyll-lcsb:1.6
+image: $CI_REGISTRY/r3/docker/jekyll-lcsb:1.6
 
 stages:
   - build