I think It would be much better in the long run if you were to compile a wiki first, howto and bestpractices for the masses on your blog and then try to apply your cumulative knowledge and expertise into an alpha(beta) book as the technology itself matures. While I understand your passion about docker and you want to contribute while making money, I would also think about the audience who invests time and their money on a topic that is constantly evolving.
Why is this ad on HN front page ?
OP here. Just came across it and I found it was interesting enough as there's not so much books about Docker.
There aren't a lot of books about Docker because by the time they get published, Docker has made so many breaking changes, they are irrelevant.
While you're definitely right when it comes to printed books, self-published books tend to be pretty up-to-date (as an example, I believe "Agile Web Development With Rails" runs a test suite against Rails and the author gets notified the moment something's not accurate with the book, maybe it's the case for this one).