Add a gitlab-ci build description.

Build the static site on a continuous integration runner.
This provides both testing feedback on changes and simplifies
deployment.

Copied from the opus-website repository.

.gitlab-ci.yml

+build:
+  image: jekyll/jekyll:4
+  script: jekyll build
+  artifacts:
+    paths:
+      - _site
+  tags:
+    - docker