fix: download caddy binary in CI for validate step

.woodpecker/edge-subpath.yml

@@ -122,10 +122,13 @@ steps:
   #   0 — configuration is valid
   #   1 — configuration has errors
   - name: caddy-validate
-    image: caddy:2-alpine
+    image: alpine:3.19
     commands:
-      - caddy version
-      - caddy validate --config /tmp/edge-render/Caddyfile.rendered --adapter caddyfile
+      - apk add --no-cache ca-certificates
+      - curl -sS -o /tmp/caddy "https://caddyserver.com/api/download?os=linux&arch=amd64"
+      - chmod +x /tmp/caddy
+      - /tmp/caddy version
+      - /tmp/caddy validate --config /tmp/edge-render/Caddyfile.rendered --adapter caddyfile
 
   # ── 4. Caddyfile routing block shape test ─────────────────────────────────
   # Verify that the Caddyfile contains all required routing blocks: