chore: add reproducible docker creation description

README.org

@@ -10,7 +10,7 @@ The presentation can be found [[https://gitlab.ost.ch/sml-cs/reading-events/1st-
 The container was created with
 
 #+begin_src bash
-  archive = $(guix pack -f docker -S /bin=bin -S /lib=lib -S /share=share -m manifest.scm)
+  archive = $(guix time-machine -C channels.scm -- pack -f docker -S /bin=bin -S /lib=lib -S /share=share -m manifest.scm)
   docker load < $archive
   docker tag bash-coreutils-git-emacs registry.gitlab.ost.ch:45023/sml-cs/reading-events/1st-reading-event
   docker push registry.gitlab.ost.ch:45023/sml-cs/reading-events/1st-reading-event