Edited;
Ok, I just managed to run the docker container conencted to the created networrk.
I had to run
$ docker rm bitcoind
$ docker run -it --network lnbook --name bitcoind lnbook/bitcoind
That's correct; even if you stopped the docker container, it still
'existed' and you therefore need to delete / remove it (rm).
If you followed the book step by step, I'd honestly consider this a 'bug' / error and would recommend to create an
issue with description and fix (those 2 terminal commands).