What is the primary purpose of the EXPOSE command in a Dockerfile?

The primary purpose of the EXPOSE command in a Dockerfile is to declare which ports the container listens on at runtime. This command is an important part of specifying how a Docker container interacts with the outside world. By indicating the ports that the application inside the container is using, you provide documentation for users of the image and help other developers or system administrators understand which ports they need to publish when running the container.

When the EXPOSE command is used, it does not actually map the ports to the host machine; instead, it serves as a hint for users so they know which ports need to be exposed during container deployment. For instance, when running a web application inside a Docker container, specifying the relevant port with EXPOSE makes it clear that this port will need to be opened to allow traffic.

In contrast, the other options do not correctly describe the function of the EXPOSE command. Creating a new layer in the Docker image pertains to the filesystem changes made by other commands such as RUN or COPY. Setting environment variables is typically done using the ENV command, and selecting the base image is specified with the FROM command in the Dockerfile. Thus, declaring the ports the container listens on is the essential role of the EXPOSE command.