build: document driver and image store support for attestations

[davidpavlovschi]

Description

The build attestations page states that "All BuildKit exporters support attestations", but never mentions that the default docker driver rejects attestations while the daemon still uses the classic image store. That combination is what Docker Engine 29 errors on, and the error's Learn more link (/go/attestations/, mapped to /build/attestations/ in data/redirects.yml:222-223) lands on exactly this page, which today has no answer.

This adds a Driver and image store support subsection under Creating attestations in content/manuals/build/metadata/attestations/_index.md:

• Attestations attach to an image index, and the classic image store doesn't support image indices, so it can't hold an image with attestations.
• With the default docker driver on the classic image store, the build fails; the exact engine error string is quoted.
• How to fix it: turn on the containerd image store, with links to /manuals/desktop/features/containerd.md and /manuals/engine/storage/containerd.md.
• A compact 3-row driver support table (docker, docker-container, kubernetes and remote).
• A note that docker-container, kubernetes, and remote run BuildKit outside the daemon, that --push keeps attestations, and that --load runs into the same image store requirement.
• An opt-out line (--provenance=false --sbom=false).

The local/tar exporter behavior is already covered under Storage, so that section is untouched. One file changed, 33 insertions, no deletions.

Sources

Both claims are already documented elsewhere in this repository and are reused verbatim rather than invented:

• content/manuals/desktop/features/containerd.md (Classic image store): "It doesn't support image indices or manifest lists, so you can't load multi-platform images locally or build images with attestations."
• content/manuals/dhi/how-to/scan.md:225: "Without the containerd image store, Docker Engine rejects the build with: Attestation is not supported for the docker driver. Switch to a different driver, or turn on the containerd image store, and try again." — the error text in the new section is copied from this in-repo string, matching the one in the issue report.
• Driver descriptions match content/manuals/build/builders/drivers/_index.md ("docker: uses the BuildKit library bundled into the Docker daemon").

Validation

• markdownlint-cli@0.45.0 (the version pinned in the repo Dockerfile lint stage) on the changed file: 0 issues.
• vale 3.16.0 with the repo's .vale.ini and _vale/ styles on the changed file: 0 errors, 0 warnings, 0 suggestions.
• prettier@3.8.1 (repo-pinned version) check on the changed file: passes, formatting unchanged elsewhere in the file.
• Both new link targets exist in the tree: content/manuals/desktop/features/containerd.md, content/manuals/engine/storage/containerd.md.
• htmltest and the Hugo build were not run locally (they require the Docker-based bake targets, and no daemon was available in this environment).

Related issues or tickets

Fixes #25599

Reviews

• Technical review
• Editorial review
• Product review

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