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

The ENTRYPOINT instruction in a Dockerfile is crucial for defining what command will be executed when a container is started from the image that the Dockerfile creates. This instruction establishes the main process that runs in the container, ensuring that the specified application or script is executed upon container launch. By using ENTRYPOINT, users can setup the container to behave like a standalone executable, making it intuitive for others to run the container with default behavior.

While there are different ways to specify commands in Dockerfiles, using ENTRYPOINT provides flexibility and control over how the command is executed. It can also be used in conjunction with the CMD instruction, which can supply default arguments to the ENTRYPOINT command. This means that the ENTRYPOINT can be fixed, while CMD allows some customization by users when they run the container.

In contrast, setting environment variables, exposing ports, or creating a Docker image from the Dockerfile are separate functionalities that serve distinct purposes in containerization and image management. Understanding the role of each instruction helps in effectively building and managing Docker containers.