Mirror Minio binary builds

[arschles]

As seen in the fix in #106, the Minio download links can change, and when they do, our builds fail. After #106, they will "go red" (as opposed to the former behavior, in which they failed silently), but it would be even better for us to mirror the binary builds on our own S3 or GCS bucket, so that we control the download URLs.

This issue is for copying binaries into the S3/GCS bucket that we own, and changing the Dockerfile to point to that bucket.