Fix or replace gitbook (in Dockerfile)

[drwetter]

Just building a new PDF since a while and I was confronted with this ugly message:

Probably the threat is not high. But for a secuirty project it's just ugly. Can you @PauloASilva or sombody else give a hand here?
Gitbook-cli supported is is no longer under active development. as stated here: https://github.com/GitbookIO/gitbook