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/profiledef.sh

@@ -9,8 +9,7 @@ iso_version="$(date +%Y.%m)"
 install_dir="arch"
 buildmodes=('iso')
 bootmodes=('bios.syslinux.mbr' 'bios.syslinux.eltorito'
-           'uefi-ia32.grub.esp' 'uefi-x64.grub.esp'
-           'uefi-ia32.grub.eltorito' 'uefi-x64.grub.eltorito')
+           'uefi-x64.systemd-boot.esp' 'uefi-x64.systemd-boot.eltorito')
 arch="x86_64"
 pacman_conf="pacman.conf"
 airootfs_image_type="squashfs"