What does the `EXPOSE` instruction in a Dockerfile signify?

The EXPOSE instruction in a Dockerfile is used to indicate which network ports the container will listen on when it is running. This is important for inter-container communication and allows users to understand which ports should be mapped or accessed on the host machine. By documenting the ports that the application inside the container uses, it reduces confusion and enhances the usability of the container image.

Although the EXPOSE instruction itself does not actually publish the ports or make them accessible from outside the container (that would be done with the -p flag when running a container), it serves as a form of documentation for anyone using the image or for container orchestration systems. This instruction makes it clear to developers and operations teams which ports are intended for incoming traffic, ensuring better organization and architecture of their containerized applications.