Fix ARM64 boot: install official archiso mkinitcpio hooks into airootfs

The archiso mkinitcpio hooks live in initcpio/ at the repo root, not in
configs/releng/airootfs/ — so they were never in the ISO airootfs chroot.
ALARM has no archiso package either, so the chroot's mkinitcpio found no
archiso hook, built an initramfs without mount_handler logic, and the
default handler fired at boot with empty root= → "Failed to mount ''".

Dockerfile.arm64: preserve the official hooks from initcpio/ into
/usr/share/archiso/initcpio/ instead of deleting them with the repo.

entrypoint-arm64.sh: explicitly install the official hooks into the
airootfs overlay so the chroot mkinitcpio finds them when building the
initramfs. The official hooks use uname -m for arch detection and include
all required modules/binaries for squashfs live boot.

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

iso/docker/Dockerfile.arm64

@@ -56,13 +56,17 @@ RUN pacman -Syu --noconfirm && \
     && pacman -Scc --noconfirm
 
 # Install archiso from upstream source (not packaged for aarch64 in ALARM).
-# Manually copy the three things mkarchiso needs — skip 'make install' because
-# it tries to build man pages with rst2man which isn't available.
+# Manually copy what mkarchiso needs — skip 'make install' (requires rst2man).
+# Also preserve the mkinitcpio hooks from initcpio/ so the entrypoint can
+# install them into the ISO airootfs (the chroot has no archiso package, so
+# the hooks must come from the airootfs overlay for mkinitcpio to use them).
 RUN git clone --depth=1 https://gitlab.archlinux.org/archlinux/archiso.git /tmp/archiso && \
-    install -Dm755 /tmp/archiso/archiso/mkarchiso   /usr/bin/mkarchiso && \
-    install -Dm755 /tmp/archiso/scripts/run_archiso.sh /usr/bin/run_archiso && \
+    install -Dm755 /tmp/archiso/archiso/mkarchiso        /usr/bin/mkarchiso && \
+    install -Dm755 /tmp/archiso/scripts/run_archiso.sh   /usr/bin/run_archiso && \
     install -d /usr/share/archiso && \
     cp -a /tmp/archiso/configs /usr/share/archiso/ && \
+    install -Dm644 /tmp/archiso/initcpio/hooks/archiso   /usr/share/archiso/initcpio/hooks/archiso && \
+    install -Dm644 /tmp/archiso/initcpio/install/archiso /usr/share/archiso/initcpio/install/archiso && \
     rm -rf /tmp/archiso
 
 # Pre-download linux-aarch64 and create a local pacman repo so mkarchiso