chore(deps): update docker/metadata-action action to v6 (#68)

This PR contains the following updates:

| Package | Type | Update | Change |
|---|---|---|---|
| [docker/metadata-action](https://github.com/docker/metadata-action) | action | major | `v5` → `v6` |

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### Release Notes

<details>
<summary>docker/metadata-action (docker/metadata-action)</summary>

### [`v6`](https://github.com/docker/metadata-action/compare/v5...v6)

[Compare Source](https://github.com/docker/metadata-action/compare/v5...v6)

</details>

---

### Configuration

📅 **Schedule**: Branch creation - At any time (no schedule defined), Automerge - At any time (no schedule defined).

🚦 **Automerge**: Disabled by config. Please merge this manually once you are satisfied.

♻ **Rebasing**: Whenever PR is behind base branch, or you tick the rebase/retry checkbox.

🔕 **Ignore**: Close this PR and you won't be reminded about this update again.

---

 - [ ] <!-- rebase-check -->If you want to rebase/retry this PR, check this box

---

This PR has been generated by [Renovate Bot](https://github.com/renovatebot/renovate).
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Reviewed-on: #68
Co-authored-by: renovate[bot] <renovate-bot@git.tainton.uk>
Co-committed-by: renovate[bot] <renovate-bot@git.tainton.uk>

.github/workflows/build-push-attest-docker.yml

@@ -40,7 +40,7 @@ jobs:
 
       - name: Extract metadata (tags, labels) for Docker
         id: meta
-        uses: docker/metadata-action@v5
+        uses: docker/metadata-action@v6
         with:
           images: |
             ghcr.io/${{ github.repository }}