switch to Caddy reverse proxy with auto HTTPS

Replace manual nginx + SSL certificates with Caddy for automatic
Let's Encrypt certificate management. Remove direct HTTP port exposure,
route web traffic through Caddy (80/443) via Docker internal network.
SSH remains on port 2222.

Caddyfile

@@ -0,0 +1,3 @@
+{$DOMAIN} {
+	reverse_proxy gitea:3000
+}