Merge branch 'change-baseimg' into 'develop'

change base img

See merge request R3/howto-cards!142

.gitlab-ci.yml

 # In case something goes horribly wrong, you can fall back to `image: ruby:2.5`
-image: git-r3lab.uni.lu:4567/r3/docker/jekyll-lcsb:1.5
+image: git-r3lab.uni.lu:4567/r3/docker/jekyll-lcsb:1.6
 
 stages:
   - build