compose: note NET_RAW cap for ICMP probe; drop Dockerfile syntax pin

Dockerfile

@@ -1,5 +1,3 @@
-# syntax=docker/dockerfile:1.7
-
 FROM oven/bun:1-alpine AS frontend
 WORKDIR /ui
 COPY package.json bun.lock ./

docker-compose.yml

@@ -11,5 +11,6 @@ services:
       - CONFIG=/app/config.local.yaml
       - ADDR=:5000
     cap_add:
+      # required for the ICMP ping probe (raw socket); Docker drops NET_RAW by default
       - NET_RAW
     restart: unless-stopped