ARM64: add boot diagnostics + pre-cache linux-aarch64 in Docker image

- Add ls/pacman -Q diagnostics in customize_airootfs.sh so we can see
  exactly what is (or isn't) in /boot after pacstrap
- Pre-download linux-aarch64 in Dockerfile with pacman -Sw so mkarchiso
  can install it from the local pacman cache even if the [alarm] mirror
  is unreachable at ISO-build time

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

iso/docker/entrypoint-arm64.sh

@@ -643,6 +643,14 @@ locale-gen
 echo "LANG=en_US.UTF-8" > /etc/locale.conf
 ln -sf /usr/share/zoneinfo/UTC /etc/localtime
 
+echo "=== /boot/ contents ==="
+ls -la /boot/ 2>&1 || echo "(cannot list /boot)"
+echo "=== linux-aarch64 package status ==="
+pacman -Q linux-aarch64 2>&1 || echo "NOT INSTALLED"
+echo "=== /usr/lib/modules/ ==="
+ls /usr/lib/modules/ 2>&1 || echo "(empty)"
+echo "=== end diagnostics ==="
+
 useradd -m -G wheel,audio,video,storage,input -s /bin/bash orion 2>/dev/null || true
 echo "orion:orionos" | chpasswd