Dry-run for update-wb.

Download the new image but don't create a new container (safe to run in
the background).

Documents/bin/update-wb

@@ -23,6 +23,9 @@ update() {
 if [ "$(hostname)" = 'toolbox' ]
 then
     /usr/libexec/flatpak-xdg-utils/flatpak-spawn --host "$0" "$@"
+elif [ "$1" = "-d" ] || [ "$1" = "--dry-run" ] || [ -n "${DRY_RUN:-}" ]
+then
+    podman image pull "$IMAGE" > /dev/null
 elif ! (is_latest)
 then
     update