chore(deps): update docker/build-push-action action to v6 (#32)

This PR contains the following updates:

| Package | Type | Update | Change |
|---|---|---|---|
| [docker/build-push-action](https://github.com/docker/build-push-action) | action | major | `v4` -> `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)

### [`v5`](https://github.com/docker/build-push-action/compare/v4...v5)

[Compare Source](https://github.com/docker/build-push-action/compare/v4...v5)

</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 becomes conflicted, 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

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This PR has been generated by [Renovate Bot](https://github.com/renovatebot/renovate).
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Reviewed-on: #32
Co-authored-by: Renovate Bot <renovate@t00n.de>
Co-committed-by: Renovate Bot <renovate@t00n.de>

.gitea/workflows/cd.yaml

@@ -92,7 +92,7 @@ jobs:
           echo REPO_NAME=$(echo ${GITHUB_REPOSITORY} | awk -F"/" '{print $2}' | tr '[:upper:]' '[:lower:]') >> $GITHUB_OUTPUT
           echo REPO_VERSION=$(git describe --tags --always | sed 's/^v//') >> $GITHUB_OUTPUT
       - name: Build and push
-        uses: docker/build-push-action@v4
+        uses: docker/build-push-action@v6
         with:
           context: .
           file: ./Dockerfile