panic: nil deref in createVCS() scanning multi-arch image by tag when no attestation sidecar exists

Describe the bug

Running docker scout cves : against a multi-arch image index (manifest list) with no attestation sidecar panics with a nil pointer dereference in createVCS(). Scout falsely reports ✓ Provenance obtained from attestation before crashing — even though docker buildx imagetools inspect --format '{{ json .Provenance }}' returns blank/null, confirming no provenance exists.

Scanning the exact same image by digest succeeds. The bug is specific to tag-based resolution of multi-arch image indexes with no unknown/unknown attestation entry.

This persists on v1.21.0 despite "Fix VCS provenance parsing" being listed in the v1.20.0+ changelog.

To Reproduce

Use any multi-arch image (manifest list) built without a provenance attestation sidecar — i.e. no unknown/unknown platform entry in the image index.

Fails (tag-based):

docker scout cves registry.example.com/project/nginx:1.0.0

Succeeds (digest-based, same image):

docker scout cves registry.example.com/project/nginx@sha256:

Confirm no attestation is present:

docker buildx imagetools inspect registry.example.com/project/nginx:1.0.0 \
  --format '{{ json .Provenance }}'
# returns blank / null output

docker buildx imagetools inspect registry.example.com/project/nginx:1.0.0
# shows only linux/amd64 and linux/arm64 manifests — no unknown/unknown attestation entry

Expected behavior

Scout detects no provenance attestation and skips provenance parsing, completing the CVE scan from the SBOM alone. It should not report ✓ Provenance obtained from attestation when none exists.

Actual behavior

    ✓ SBOM obtained from attestation, 30 packages found
    ✓ Provenance obtained from attestation
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x28 pc=0x2ee7dde]

goroutine 1 [running]:
github.com/docker/scout-cli-plugin/internal/attestations.createVCS(0x18df7bfb26e0)
        /home/runner/work/scout-cli-plugin/scout-cli-plugin/internal/attestations/provenance.go:128 +0x1e
github.com/docker/scout-cli-plugin/internal/attestations.(*Attestations).Provenance(0x18df7c12a900)
        /home/runner/work/scout-cli-plugin/scout-cli-plugin/internal/attestations/provenance.go:117 +0xb3
github.com/docker/scout-cli-plugin/internal/attestations.(*Attestations).CombineWith(0x18df7c12a900, 0x18df7b28c700)
        /home/runner/work/scout-cli-plugin/scout-cli-plugin/internal/attestations/attestations.go:155 +0x105
github.com/docker/scout-cli-plugin/sbom.(*Service).FromImageAttestation(0x5?, {0x7995ac8?, 0x18df7bd0b500?}, 0x52c899?, ...)
        /home/runner/work/scout-cli-plugin/scout-cli-plugin/sbom/sbom.go:270 +0xa5
github.com/docker/scout-cli-plugin/sbom.(*Service).FromImage(...)
        /home/runner/work/scout-cli-plugin/scout-cli-plugin/sbom/sbom.go:161 +0x327
github.com/docker/scout-cli-plugin/sbom.(*Service).Get(...)
        /home/runner/work/scout-cli-plugin/scout-cli-plugin/sbom/sbom.go:124 +0x597
github.com/docker/scout-cli-plugin/internal/commands/cves.NewCmd.func2(...)
        /home/runner/work/scout-cli-plugin/scout-cli-plugin/internal/commands/cves/cves.go:234 +0x7d7

Workarounds

1. Scan by digest: docker scout cves registry.example.com/project/nginx@sha256:
2. Skip provenance parsing: docker scout cves --only-sbom registry.example.com/project/nginx:1.0.0

Root cause hypothesis

When resolving a tag pointing to a multi-arch image index, scout's attestation detection at the index level returns a false positive — it signals provenance was found when the manifest list contains no unknown/unknown attestation sidecar. A nil/empty provenance object is then passed into CombineWith() → Provenance() → createVCS() at provenance.go:128 without a nil guard, causing the panic.

Two issues compound here: (1) false positive attestation detection on manifest lists, and (2) missing nil guard in createVCS(). Both should be fixed independently.

Environment

• Scout version: v1.21.0 (go1.26.3 - linux/amd64)
• Docker CLI: v29.5.0
• OS: Linux amd64

Start with internal/attestations/provenance.go at createVCS() and trace the Provenance() and CombineWith() calls shown in the stack trace. Reproduce the tag-based scan with a multi-arch image lacking an unknown/unknown attestation, then compare it with digest-based resolution and --only-sbom. Done means the scan skips provenance parsing when none exists, avoids the panic, and does not report provenance obtained.