Weird. I just re-ran what I have and it did work.
I used GitHub - davetron5000/sidekiq-book: Starting App used for the Sidekiq book which should be identical to what’s in the book and what was packaged. Is that what you are using?
Of note, I’m on an M3 MacBook.
If what you have is not exactly like the repo above, can you post your Dockerfile.dx
and I can see what the issue is?