use main docker images

[ylebre]

This lets phpunit run using the docker images from 'main' instead of the PR branch 179.

Fixes #196

[Potherca]

To allow the GitHub Action (GHA) to build/push docker images to a fork, the hard-codded pdsinterop/solid-nextcloud has been replaced with the github.repository context variable in the relevant GHAs.

These changes can be seen working on my fork: https://github.com/potherca-contrib/solid-nextcloud

Steps taken:

1. Merge the MR
2. Wait for GHA to run
3. Check that Docker images have been created fromthe merged MR
4. Re-run the failed PHPUnit GHA.

After that, all checks are green: https://github.com/potherca-contrib/solid-nextcloud/actions/runs/15396265579