chore(deps): update docker.io/library/nginx docker tag to v1.21.0

This MR contains the following updates:

Package	Type	Update	Change
docker.io/library/nginx	final	minor	1.20.0-alpine -> 1.21.0-alpine

---

Configuration

📅 Schedule: At any time (no schedule defined).

🚦 Automerge: Enabled.

♻️ Rebasing: Whenever MR becomes conflicted, or you tick the rebase/retry checkbox.

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

---

• If you want to rebase/retry this MR, check this box.

---

This MR has been generated by Renovate Bot. The local configuration can be found in the SI Renovate Bot repository.