Final status update
docker image build -t gforghetti/wordpress:latest . is incorrect - (the word image should be removed), so the correct version is:
docker build -t gforghetti/wordpress:latest .
(I used a different tag, of course).
When I started this new app via
docker compose, I found this situation:
Once I added all missing plugins, I was also able to add the plugin that previously would not load (because of the 2M limit).
In summary, your last solution works just fine and I am not sure why all of my plugins disappeared. If you have any explanation for this, it would make me very happy.
Thanks for all efforts spent on this!