Kubernetes is not the only system that can be used to cluster Docker containers. Even though it's currently the most popular one, there may be some valid reasons to use different software. Let's walk-through the alternatives.
Docker Swarm is a native clustering system for Docker that turns a set of Docker hosts into one consistent cluster, called a swarm. Each host connected to the swarm plays the role of a manager or a worker (there must be at least one manager in a cluster). Technically, the physical location of the machines does not matter; however, it's reasonable to have all Docker hosts inside one local network, otherwise managing operations (or reaching a consensus between multiple managers) can take a significant amount of time.