Attempt fixing build errors

.gitlab-ci.yml

 # ruby:2.6.4 (ruby:latest at time of writing has bundle version 1.17.2, which is too old)
 #image: ruby:latest
-image: ruby:rc
+image: ruby:rc-buster
 
 variables:
   JEKYLL_ENV: production
@@ -8,6 +8,8 @@ variables:
 
 before_script:
   - bundle install
+  - apt install git -y
+  - git submodule update --init --recursive
 
 test:
   stage: test