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/packages.x86_64

@@ -24,10 +24,12 @@ libxft
 libxfixes
 libxext
 libpng
+imlib2
 dbus
-libpam
+pam
 fontconfig
 freetype2
+syslinux
 
 # Desktop utilities
 feh
@@ -40,14 +42,13 @@ alsa-utils
 pulseaudio
 
 # Brightness
-light
+brightnessctl
 
 # Media playback control
 playerctl
 
 # Arch tools
 pacman-contrib
-pkexec
 polkit
 
 # System tools