What is the purpose of the CMD instruction in a Dockerfile?

The CMD instruction in a Dockerfile is designed to specify the default command that should be executed when a container is run from the Docker image. This command can be a single command or an array of commands that will be run automatically when the container starts. It effectively tells Docker what operation to perform when the container is launched, serving the purpose of defining the container's behavior in terms of its primary function.

When a container is created by executing a Docker image, the command specified by CMD is what gets executed in the container environment. If no command is provided by the user at the time of running the container, the command defined by CMD will be used. It's important to note that CMD can also be overridden by passing a different command when starting the container, providing flexibility in container behavior based on user requirements.

The other options do not accurately describe the function of the CMD instruction: setting the working directory is done using the WORKDIR instruction, defining build arguments is accomplished with ARG, and specifying the version of the Dockerfile is not directly relevant as Dockerfile versions are indicated by the syntax and structure of the file itself rather than a specific instruction within it.