Which of the following best describes a Docker container?

A Docker container is best described as a running instance of a Docker image. This definition captures the nature of containers in the Docker ecosystem, highlighting their ephemeral and isolated nature. When a Docker image, which is a packaged set of application code and all its dependencies, is executed, it creates a container. This container runs in its own environment, yet shares the underlying operating system kernel with other containers, which allows it to operate more efficiently than traditional virtual machines.

The concept of a container encapsulates everything needed to run an application but does not include a full operating system. Instead, containers rely on the host OS, making them lightweight. This is different from a virtual machine, which includes a full operating system along with the application, resulting in heavier resource usage. Additionally, a configuration file does not adequately describe a container; while Docker uses Dockerfiles to automate the image creation process, the container is ultimately an instantiated image, not just a set of instructions.