Update Bufstream examples using MinIO to use alpine image

[jrinehart-buf]

• Updates the two examples using MinIO (Docker Compose and Iceberg) to:
  • Use the Alpine image of MinIO because MinIO ceased publishing public Docker images in October 2025.
  • Use the AWS-provided AWS CLI docker image instead of MinIO control. It's less lines of code and a more familiar example.
• Updates the Iceberg example to use current network config and, consistently with the Compose example, use Postgres as its metadata store. This also makes its data durable across Docker up/down.