Docker is terrific for standing up an instance of software, but container networking can be a bytch and is sometimes difficult to wrap your head around in the context of machine networking. I can't speak to the cert you're going for, but my experience with Docker networking normally is making sure ports are exposed and linked to a host port. Never had to touch any subnets or IPs, or any stuff that I associate with networking. I'm sure it can be done, and it's probably useful knowledge, but like I mentioned it's upfront learning you have to put in before you get to what you're wanting to learn.
Now I'm not in the cert world, but I've heard that books and some courses will cover areas not on the test. So do your book studying in conjunction with practice tests, if you get your hands on some.