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/Dockerfile.arm64

@@ -65,6 +65,10 @@ RUN git clone --depth=1 https://gitlab.archlinux.org/archlinux/archiso.git /tmp/
     cp -a /tmp/archiso/configs /usr/share/archiso/ && \
     rm -rf /tmp/archiso
 
+# Pre-download linux-aarch64 so mkarchiso can install it from local cache
+# even if the [alarm] mirror is unreliable at ISO-build time.
+RUN pacman -Sw --noconfirm linux-aarch64
+
 WORKDIR /orionos
 COPY . .