Pull MinIO from pgsty/silo instead of building it from source

[sgamelin]

Building MinIO and mc from source (#4) makes every docker compose up download and compile about 241 Go modules. That's about 2 minutes on a typical runner and up to 9½ on a slow one. On 2026-09-29 it also failed nisshi-io/nisshi's merge queue: all three failed smoke legs died in go build on stream error: INTERNAL_ERROR from proxy.golang.org (run).

This switches the minio service to pgsty/silo, a community-maintained AGPLv3 fork of MinIO that still publishes images, and removes etc/minio/Dockerfile.

• Pinned to RELEASE.2026-09-16T00-00-00Z by index digest. It covers linux/amd64 and linux/arm64, the two runner types in this matrix.
• The image has mc at /usr/bin/mc and has bash, so the compose healthcheck, the mc steps in ci.yml, the justfile recipes and bin/up run unchanged. Its entrypoint maps the existing server /data ... command to its silo binary.
• Pulling a public image means we don't redistribute MinIO ourselves, so there are no AGPL obligations on our side, unlike publishing our own build.

Verification

Locally, against nisshi-io/nisshi's identical compose service:

• The image is 52 MB and pulled in about 3 s.
• The compose healthcheck, mc ready local, mc alias set and mc mb local/nisshi all pass.
• Conditional writes, which nisshi's S3 engine relies on, work: a second PutObject with If-None-Match: * and one with a wrong If-Match both return PreconditionFailed, and the correct If-Match succeeds.
• ghcr.io/nisshi-io/nisshi:main on --storage-engine s3://nisshi/ created a topic and produced 500 idempotent records, and a consumer group read all 500 back.

The s3://nisshi/ legs of this PR's CI run the full compose stack and the bats suite against silo.

Expected failures: the postgres:// legs

They fail on main too (run). The cause is the etc/initdb.d/010-schema.sql drift described in #4, not this change.

Follow-up

nisshi-io/nisshi's smoke job pins this repo by commit. Once this merges, a nisshi PR will move that pin to the merge commit, alongside the same compose change in nisshi.

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