chore(deps): update docker/setup-buildx-action action to v4

This PR contains the following updates:

| Package | Type | Update | Change |
|---|---|---|---|
| [docker/build-push-action](https://github.com/docker/build-push-action) | action | major | `v6` → `v7` |

---

### Release Notes

<details>
<summary>docker/build-push-action (docker/build-push-action)</summary>

### [`v7`](https://github.com/docker/build-push-action/compare/v6...v7)

[Compare Source](https://github.com/docker/build-push-action/compare/v6...v7)

</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).
<!--renovate-debug:eyJjcmVhdGVkSW5WZXIiOiI0My41Ni4xIiwidXBkYXRlZEluVmVyIjoiNDMuNTYuMSIsInRhcmdldEJyYW5jaCI6Im1haW4iLCJsYWJlbHMiOlsidHlwZS9kZXBlbmRlbmNpZXMiXX0=-->

Reviewed-on: #45
Reviewed-by: Luke Tainton <luke@tainton.uk>
Co-authored-by: renovate[bot] <renovate-bot@git.tainton.uk>
Co-committed-by: renovate[bot] <renovate-bot@git.tainton.uk>

.gitea/workflows/release.yml

@@ -51,7 +51,7 @@ jobs:
           ref: ${{ needs.tag.outputs.tag_name }}
 
       - name: Set up Docker Buildx
-        uses: docker/setup-buildx-action@v3
+        uses: docker/setup-buildx-action@v4
 
       - name: Log in to Gitea Container Registry
         uses: docker/login-action@v4
@@ -85,7 +85,7 @@ jobs:
           printf "Tags:\n${{ steps.meta.outputs.tags }}"
 
       - name: Build images
-        uses: docker/build-push-action@v6
+        uses: docker/build-push-action@v7
         with:
           context: .
           push: false