Create /dev/null in new root to prevent hangs

gnupg post install script does "dirmngr </dev/null &>/dev/null".
Since "pacman -r" does not mount any filesystems but it does chroot
into the new root, /dev/null does not exist, making the script hang
forever.

pacstrap-docker

@@ -19,7 +19,13 @@ mkdir -m 0755 -p "$newroot"/var/{cache/pacman/pkg,lib/pacman,log} "$newroot"/{de
 mkdir -m 1777 -p "$newroot"/tmp
 mkdir -m 0555 -p "$newroot"/{sys,proc}
 
+echo 'Creating /dev/null in new root'
+mknod "$newroot/dev/null" c 1 3
+
 echo 'Installing packages to %s' "$newroot"
 if ! pacman -r "$newroot" -Sy --noconfirm "${pacman_args[@]}"; then
 	die 'Failed to install packages to new root'
 fi
+
+echo 'Deleting /dev/null from new root'
+rm "$newroot/dev/null"