docs: the attestations are on the platform image, not the index

Checked against netork's first signed images (main-142912c): cosign verify
on the tag succeeds, verify-attestation on it finds only the signature,
because BuildKit pushes even one platform as an index with its provenance
manifest. The CIS result and SBOM are on the linux/amd64 image.

README.md

@@ -66,11 +66,19 @@ pinned and checked against its release checksum (`bin/install-cosign.sh`).
 ## Verifying an image yourself
 
 ```bash
-cosign verify --key cosign.pub --insecure-ignore-tlog registry.netork.io/netork/base-os@sha256:…
+# The signature, on the tag or digest you pull (an index is signed with every image in it):
+cosign verify --key cosign.pub --insecure-ignore-tlog registry.netork.io/netork/engine:latest-dev
+
+# The CIS result and the SBOM belong to each platform image, not to the index around it:
+docker buildx imagetools inspect registry.netork.io/netork/engine:latest-dev   # the linux/amd64 digest
 cosign verify-attestation --key cosign.pub --insecure-ignore-tlog \
-  --type https://netork.io/attestations/cis/v1 registry.netork.io/netork/base-os@sha256:…
+  --type https://netork.io/attestations/cis/v1 registry.netork.io/netork/engine@sha256:<platform digest>
+cosign verify-attestation --key cosign.pub --insecure-ignore-tlog \
+  --type cyclonedx registry.netork.io/netork/engine@sha256:<platform digest>
 ```
 
+Even a single-platform image is pushed as an index: BuildKit adds its provenance manifest.
+
 `cosign.pub` is the public half of the `COSIGN_PRIVATE_KEY_B64` organization secret. The private
 key comes base64 in one line: the runner masks a secret only where it appears whole.