Use new debian image

This merge request fixes some issues that showed up after updating the ci-website-builder image, see #97 (closed), !398 (merged), !399 (merged). After the update of the image to a newer fedora image lead to strange errors while building the website. For that reason we replaced the fedora image with a debian image.

This merge request adjusts the dune-website accordingly by removing the entrypoint from gitlab-ci.yml.