chore(deps): update docker/build-push-action action to v6 (#34)

This PR contains the following updates:

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

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### Release Notes

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

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

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

</details>

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### 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: https://git.mgrote.net///container-images/postfix/pulls/34
Co-authored-by: Renovate Bot <renovate@mgrote.net>
Co-committed-by: Renovate Bot <renovate@mgrote.net>

.gitea/workflows/build.yaml

@@ -57,7 +57,7 @@ jobs:
         run: echo "REPO_BRANCH=${GITHUB_HEAD_REF:-${GITHUB_REF#refs/heads/}}" >> $GITHUB_OUTPUT
         id: extract_branch
       - name: Build and push Docker image
-        uses: https://github.com/docker/build-push-action@v5
+        uses: https://github.com/docker/build-push-action@v6
         with:
           context: .
           file: ./Dockerfile