Docker is a solution to one specific problem: the need for "a user" to run 10 different potentially conflicting apps, all at the same time, on one machine, and abstract away anything which might make those apps conflict if they ran on a single OS. It provides a dozen different solutions in one package.

K8s is a way to take that and make it scale up for a large number of applications on a large number of hosts in a production business in a way that's automated and resilient to failure.