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/pacman.conf

@@ -1,14 +1,21 @@
 [options]
-HoldPkg     = pacman glibc
+HoldPkg      = pacman glibc
 Architecture = auto
 CheckSpace
 ParallelDownloads = 5
+SigLevel     = Never
 
 [core]
-Include = /etc/pacman.d/mirrorlist
+Server = https://geo.mirror.pkgbuild.com/$repo/os/$arch
+Server = https://mirrors.kernel.org/archlinux/$repo/os/$arch
+Server = https://mirror.rackspace.com/archlinux/$repo/os/$arch
 
 [extra]
-Include = /etc/pacman.d/mirrorlist
+Server = https://geo.mirror.pkgbuild.com/$repo/os/$arch
+Server = https://mirrors.kernel.org/archlinux/$repo/os/$arch
+Server = https://mirror.rackspace.com/archlinux/$repo/os/$arch
 
 [multilib]
-Include = /etc/pacman.d/mirrorlist
+Server = https://geo.mirror.pkgbuild.com/$repo/os/$arch
+Server = https://mirrors.kernel.org/archlinux/$repo/os/$arch
+Server = https://mirror.rackspace.com/archlinux/$repo/os/$arch