2 Answers
As you already have an account on ACG, you should probably check out the course Steven has mentioned.
I’ve also got a book called Docker Deep Dive. Don’t be put off by the title, the first half of the book covers all of the basics and it’s had rave reviews. https://www.amazon.com/Docker-Deep-Dive-Nigel-Poulton/dp/1521822808/ref=sr_1_1?ie=UTF8&qid=1540935052&sr=8-1&keywords=nigel+poulton
I also have a couple of courses on a different platform. Not sure if it’s frowned on here, but I’ve got a Docker and Containers: The Big Picture and a Getting Started with Docker course on pluralsight.com.
HTH