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

The purpose of the ENTRYPOINT instruction in a Dockerfile is to define the command that runs when a container starts. This command is crucial because it determines what process will be executed when the container is initiated. By using ENTRYPOINT, developers can ensure that the container starts with a specific application or script, effectively transforming the container into a specific type of application service.

This instruction is often preferred over the CMD instruction when a user wants to enforce the execution of a specific command, as it is not easily overridden by arguments or command-line input. The defined command typically encapsulates the main application logic and can accept additional parameters or flags passed during runtime, making it versatile for different deployment scenarios.

The other options relate to functionalities that are not provided by ENTRYPOINT. Default arguments and environment variable settings are managed using different instructions (like CMD for default arguments and ENV for environment variables). Additionally, specifying the base image is done at the top of the Dockerfile using the FROM instruction, which is a separate and fundamental function distinct from what ENTRYPOINT achieves.