What is the primary role of the CMD instruction in a Dockerfile?

The primary role of the CMD instruction in a Dockerfile is to set the main command that is executed when a container is started from the image. The CMD instruction provides a way to specify the default executable and its arguments when no command is provided by the user at runtime. This feature allows users to easily customize container behavior upon startup, while still allowing for overrides if a specific command is required at the time the container is run.

Using CMD is an effective way to handle default settings and can lead to simpler container usage; when the container is run, it automatically knows what command to execute, unless the user specifies a different one. This instruction ensures that the container behavior is predictable and eliminates the need for manual command input each time the container is instantiated.

When the CMD instruction is combined with the ENTRYPOINT instruction, the CMD can provide default parameters to the ENTRYPOINT command, enhancing the flexibility and usability of Docker images. This relationship is pivotal in creating efficient and easily configurable containerized applications.