Use dockerized Fuseki for PHPUnit tests

[osma]

Currently we have set up PHPUnit tests so that Fuseki is installed by the init_fuseki.sh script as a regular Java process. If necessary, the script downloads the Fuseki installation tarball from the Apache Jena download site, starts it up, and then loads the test vocabulary data from the tests/test-vocab-data directory using the s-put utility that comes with Fuseki.

For Skosmos 3, I would like to switch to a dockerized Fuseki instance instead of using a regular Java process. This would have the following benefits:

1. The same Fuseki container (together with dockerized Skosmos, both running via docker compose) can also be used for running Cypress tests
2. We can use the exact same container setup both under GitHub Actions CI and local developer machines
3. "Eat your own dogfood": it's good practice to make use of our own Dockerfiles now that we have them

Prerequisites

We should first switch to the official Fuseki Dockerfile from Apache Jena; this is being worked on in PR #1477 by @kinow

How to implement

• init_fuseki.sh needs to be rewritten so it simply runs a docker compose command instead of the current code for downloading, installing and starting up Fuseki; also, test vocabularies should be loaded using e.g. curl since we don't have s-put available anymore
• some changes to the GitHub Actions CI configuration may be necessary
• CONTRIBUTING.md and the wiki documentation for unit and integration tests should be modified accordingly (e.g. need to explain that Docker is necessary for running PHPUnit tests locally)

[kinow]

+1!!! Thanks @osma !

[osma]

Starting work on this, using the commits from PR #1477 as a starting point.

[osma]

Closed via #1496