Fix ISO build pipeline for Docker on macOS

- Disable pacman alpm sandbox in Dockerfile (seccomp blocks it in BuildKit)
- Move mkarchiso from RUN to CMD so it runs with --privileged at docker run time
- Fix output path: mkarchiso now writes to /dist (matches volume mount)
- Add syslinux + imlib2 to Docker build packages and live ISO package list
- Copy syslinux/ and efiboot/ boot configs from archiso releng into profile
- Switch deprecated UEFI grub bootmodes to uefi-x64.systemd-boot
- Fix wrong package names: libpam→pam, light→brightnessctl, remove pkexec
- Set SigLevel=Never and hardcode reliable mirrors in profile pacman.conf
- Add mkinitcpio.conf with archiso hook so initrd can mount the live squashfs

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

iso/profile/airootfs/etc/mkinitcpio.conf

@@ -0,0 +1,4 @@
+MODULES=()
+BINARIES=()
+FILES=()
+HOOKS=(base udev microcode archiso block filesystems keyboard)