What does the `CMD` instruction in a Dockerfile provide?

The CMD instruction in a Dockerfile is designed to specify the default command and its parameters that will be executed when a container is started from the image. This command can serve two primary purposes: it acts as default arguments for the ENTRYPOINT command or, if there is no ENTRYPOINT defined, it defines the command to be executed when the container is run.

When a user runs a container, if they do not provide a command in the command line, the command specified in CMD will automatically be executed. This provides flexibility and convenience when creating containerized applications, allowing the user to override the default behavior by supplying their own command if desired. It simplifies the execution of containers by specifying sensible defaults.

For instance, in a scenario where a Dockerfile might have CMD ["python", "app.py"], launching the container without specifying a command will result in Python executing the app.py script by default. This is particularly useful for making the Docker image easier to use and ensuring that it behaves predictably.