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

The ENTRYPOINT instruction in a Dockerfile is designed to specify the command that will run when a container starts. This is crucial because it defines the primary application or service to be executed within the container. By setting an ENTRYPOINT, you ensure that your container behaves as an executable, making it intuitive for users and developers who work with your container.

When you define an ENTRYPOINT, it allows you to configure the container in such a way that it can accept arguments passed during the docker run command. This can enhance flexibility since you can supply different parameters or options at runtime, while the core command remains the same.

Understanding this functionality distinguishes it from other instructions in a Dockerfile. Building Docker images relates to the FROM, RUN, and other similar instructions, while listing executable commands pertains more to the CMD instruction. Defining environment variables is handled by the ENV instruction, which serves a different purpose in configuring the container's environment rather than specifying operations to be executed upon execution.