docs: clarify Docker install — add docker.io fallback, warn against bare apt install docker-ce

https://claude.ai/code/session_01FKCW3FDjNFj6jve4niMFXH

docs/setup.md

@@ -111,8 +111,25 @@ sudo systemctl enable fail2ban --now
 
 ## 5. Install Docker
 
+> **Note:** Do **not** run `apt install docker-ce` directly — that package
+> requires Docker's own apt repository to be added first.
+> Use one of the two methods below.
+
+**Option A — Official convenience script (recommended, installs latest Docker CE):**
+
 ```bash
 curl -fsSL https://get.docker.com | sh
+```
+
+**Option B — Debian-packaged `docker.io` (older but simpler, no extra repo needed):**
+
+```bash
+sudo apt install -y docker.io
+```
+
+Then, regardless of which option you used:
+
+```bash
 sudo usermod -aG docker pi   # allow running docker without sudo
 newgrp docker                # apply group without logging out
 docker run --rm hello-world  # verify