Update caddy Docker tag to v2.8.4

This MR contains the following updates:

Package	Type	Update	Change
caddy	final	minor	2.7.6 -> 2.8.4
caddy	stage	minor	2.7.6-builder -> 2.8.4-builder

---

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 MR becomes conflicted, or you tick the rebase/retry checkbox.

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

---

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

---

This MR has been generated by Renovate Bot.