It depends. I often don't trust just being given a Docker image. I want to see the Dockerfile. Then I can decide whether or not I want to make my own based off of that Dockerfile, or use that one. So that means understanding it.
For example, if I was going to provide a MailCow hosted service, I'm not going to just build the stack and call it good if it works. As the maintainer of that service I need to know how all the components work together.
Now it could be a different story if that image/stack came with direct support. I would still have to know how my Docker Swarm/Kubernetes Cluster works, but is it much different than installing software? We have to know how the OS and networking works, but there can be a lot that the software is doing that is a black box to us. Docker just takes it up another level.