What is the purpose of EXPOSE command in Dockerfile?

When writing your Dockerfiles, the instruction EXPOSE tells Docker the running container listens on specific network ports. This acts as a kind of port mapping documentation that can then be used when publishing the ports.

EXPOSE <port> [<port>/<protocol>...]

You can also specify this within a docker run command, such as :

docker run --expose=1234 my_app

Please note that EXPOSE will not allow communication via the defined ports to containers outside of the same network or to the host machine. To allow this to happen you need to publish the ports.