Add Docker build and publish GitHub Worflow

[Potherca]

This MR adds a GitHub Worflow that builds and publishes ("pushes") a docker image of this repository to the GitHub Container Registry (ghcr.io).

A docker image is created for each push to main, for each branch that has a merge-request to main, and for every git tag.

The docker image for this branch can be seen at: https://github.com/pdsinterop/multiuser-php-solid-server/pkgs/container/multiuser-php-solid-server/884546882?tag=ci_docker-build

This MR can be squashed, to avoid multiple commits.

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Some follow-up actions that come to mind:

• Add linting for the Dockerfile
• Change tests/testsuite/run-solid-test-suite.sh to pull the docker image from the registry, instead of building it (to reduce build time)
  

  multiuser-php-solid-server/tests/testsuite/run-solid-test-suite.sh

  Line 9 in c441e79

  docker build -t solid-php -f docker/solid.Dockerfile .

• Add a matrix to build for other PHP versions than only 8.3

@ylebre Let me know if any of these (or something else) needs to be picked up. 👍