feat(compose): add minio container

[Fenrikur]

Added a MinIO container to the docker-compose.yml for local testing. To verify, run just clean; just containerize; just up in the root of the repo and wait until minio-init has completed (might take a few seconds).
If just is unavailable, try running the following in the root of the repo:

docker compose down;
docker rmi ghcr.io/eurofurence/ef-app_backend-dotnet-core:nightly;
rm appsettings.json;
cp src/Eurofurence.App.Server.Web/appsettings.sample.json appsettings.json;
docker compose build;
docker compose up;

Caution: This will replace your appsettings.json with the default for local development!