Fix Plymouth: rebuild initramfs after setting OrionOS theme

mkinitcpio runs during pacstrap (kernel install) before
customize_airootfs.sh, so the initramfs gets the Arch default
spinner theme. Add mkinitcpio -P after plymouth-set-default-theme
so the orionos theme is baked into the final initramfs.

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

iso/docker/entrypoint.sh

@@ -772,8 +772,11 @@ update-desktop-database 2>/dev/null || true
 # Fontconfig cache
 fc-cache -f 2>/dev/null || true
 
-# Plymouth: set OrionOS as the default theme (archiso runs mkinitcpio after this)
+# Plymouth: set default theme and rebuild initramfs so the theme is baked in.
+# mkinitcpio already ran during pacstrap (kernel install) before this chroot
+# script executes, so we must rebuild here or the Arch default theme is used.
 plymouth-set-default-theme orionos 2>/dev/null || true
+mkinitcpio -P 2>&1 | tail -5 || true
 
 # xscreensaver config (screen lock)
 mkdir -p /etc/skel