renovate: Update invoiceninja Docker tag to v5.5.18

This MR contains the following updates:

Package	Update	Change
invoiceninja	patch	5.5.17 -> 5.5.18

---

Configuration

📅 Schedule: Branch creation - At any time (no schedule defined), Automerge - At any time (no schedule defined).

🚦 Automerge: Enabled.

♻ Rebasing: Whenever MR is behind base branch, 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, click this checkbox.

---

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