Update docker/build-push-action action to v6 (#62)

This PR contains the following updates:

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

---

### Release Notes

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

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

[Compare
Source](https://redirect.github.com/docker/build-push-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 was generated by [Mend Renovate](https://mend.io/renovate/).
View the [repository job
log](https://developer.mend.io/github/MISW/portal-role-sync).

<!--renovate-debug:eyJjcmVhdGVkSW5WZXIiOiIzNy4zOTMuMCIsInVwZGF0ZWRJblZlciI6IjM4LjU5LjIiLCJ0YXJnZXRCcmFuY2giOiJtYWluIiwibGFiZWxzIjpbInJlbm92YXRlIl19-->

Signed-off-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>
Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

.github/workflows/docker-publish.yml

@@ -46,7 +46,7 @@ jobs:
             type=sha,format=long
       - name: Build and push Docker image
         id: build-and-push
-        uses: docker/build-push-action@v5
+        uses: docker/build-push-action@v6
         with:
           context: .
           push: true