🔒 Fix hardcoded credentials in MinIO Dockerfile

[damacus]

🎯 What: The MINIO_ROOT_USER and MINIO_ROOT_PASSWORD were hardcoded as ARGs and exposed via the MC_HOST_local ENV in the docker/minio/Dockerfile.
⚠️ Risk: Hardcoding credentials inside a Dockerfile means they are permanently baked into the image layers, leading to significant risk of unauthorized access.
🛡️ Solution: Removed the ARG and ENV parameters from docker/minio/Dockerfile. Modified docker-compose.yml to supply these parameters securely as runtime environment variables, and updated .env.example to provide placeholders.

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PR created automatically by Jules for task 120342227862314319 started by @damacus

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