doc: update README

README.org

@@ -24,8 +24,8 @@ or use the accompanying docker container
 The container was created with
 
 #+begin_src bash
-  archive = $(guix time-machine -C channels.scm -- pack -f docker -S /bin=bin -S /lib=lib -S /share=share -m manifest.scm)
-  docker load < $archive
-  docker tag bash-coreutils-git-emacs registry.gitlab.ost.ch:45023/sml-cs/reading-events/1st-reading-event
+  archive=$(guix time-machine -C channels.scm -- pack -f docker -S /bin=bin -S /lib=lib -S /share=share -m manifest.scm)
+  tag=$(docker load -i $archive)
+  docker tag ${tag#"Loaded image: "} 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
 #+end_src