Fix URLs on forked projects by setting baseURL

GitLab Pages only really handles a single branch for each project, so forking is the best way to have the proposed pages be rendered and published by Pages.

By setting baseURL from CI using $CI_PAGES_URL we can ensure that links are set properly instead of always pointing at https://www.apertis.org/ as that's quite confusing during reviews.

However, for the main project we don't want to use the default GitLab pages address, but we want to have links using the official https://www.apertis.org/ address: to the main project now sets $BASEURL as a CI/CD variable to point at https://www.apertis.org/ while forked projects will use the default based on $CI_PAGES_URL.

Rendered here: https://em.pages.apertis.org/apertis-website/

.gitlab-ci.yml

@@ -15,7 +15,10 @@ test-html:
   image: registry.gitlab.com/pages/hugo:0.62.2
   stage: generate-html
   script:
-  - hugo
+  - DEFAULT_BASEURL=${CI_PAGES_URL##http:} # don't force stuff on plain HTTP when publishing on HTTPS
+  - BASEURL=${BASEURL:-$DEFAULT_BASEURL} # set BASEURL in the CI variables to override it
+  - echo "Using base URL '$BASEURL'"
+  - hugo -b "${BASEURL}"
   artifacts:
     paths:
     - public