Motivation:

A problem suddenly appeared where CI/CD jobs started to fail.

This is (seemingly) from the 'gem install' command pulling in a newer
version of jekyll, which caused an incompatibility with one of its
dependencies (jekyll --> sass-embedded).  Although the exact nature of
the incompatibility is unclear, the install fails while building, with
the error:

    NameError: uninitialized constant JSON::Fragment

Modification:

Remove jekyll from "gem install".  Use bundler to fetch jekyll instead.
This should honour the version.

Results:

More reliable builds

The script is strongly based on a CI/CD script from HIFI.  The content
was edited by commenting out the part creating thumbnails and pulling in
content from another repo.

The original is available here:

https://codebase.helmholtz.cloud/hifis/overall/hifis.net/-/blob/master/.gitlab-ci.yml