libvirt-executor: Add comments explaining the reason behind each "tweak"

images/libvirt-executor.sh

@@ -2,15 +2,26 @@
 # shellcheck disable=SC2034,SC2154
 IMAGE_NAME="Arch-Linux-x86_64-libvirt-executor-${build_version}.qcow2"
 DISK_SIZE="40G"
+# https://docs.gitlab.com/runner/executors/custom.html#prerequisite-software-for-running-a-job
 PACKAGES=(git git-lfs gitlab-runner)
 SERVICES=()
 
 function pre() {
+  # The service is a bit slow and it is not needed for our use-case as
+  # the "hardware" clock is always in UTC.
+  # https://gitlab.archlinux.org/archlinux/arch-boxes/-/merge_requests/183
   arch-chroot "${MOUNT}" /usr/bin/systemctl disable systemd-time-wait-sync
+  # Jobs often upgrade all the packages as the first thing, but we don't
+  # want the linux package to be upgraded as that would mean that
+  # relevant kernel modules cannot be loaded.
+  # https://gitlab.archlinux.org/archlinux/packaging/packages/linux/-/issues/10
   sed -E 's/^#(IgnorePkg *=)/\1 linux/' -i "${MOUNT}/etc/pacman.conf"
+  # https://gitlab.archlinux.org/archlinux/infrastructure/-/commit/ab612463a7ea119d4f0a34e9f2730b6c79cd7691
   sed 's/^\(GRUB_CMDLINE_LINUX=".*\)"$/\1 lockdown=confidentiality"/' -i "${MOUNT}/etc/default/grub"
   arch-chroot "${MOUNT}" /usr/bin/grub-mkconfig -o /boot/grub/grub.cfg
-  # We want to use the transient hostname
+  # This is needed for our injected hostname to be used, which only
+  # happens if a static hostname is not configured.
+  # https://gitlab.archlinux.org/archlinux/infrastructure/-/commit/001300ff54d826696f2d7438063c09d2e8c9afd8
   # https://github.com/systemd/systemd/pull/30814
   rm -f "${MOUNT}/etc/hostname"