Dockerfile: why do you need EXPOS or VOLUME if you still need to forward ip or mount when starting a container?

Question

Docker

I'm studying docker, and I ran into two questions:

1. The first question follows from the title of the question: why do we need EXPOS or VOLUME, if when starting the container you still need to forward ip or make a mount?
For example (in the example I omitted unnecessary moments):

FROM ubuntu:latest
RUN apt update && apt install -y nginx && apt install -y php-fpm
EXPOSE 80

why write EXPOSE 80 if ip is still not forwarded, and you will need to specify -p 80:80 when starting the container ?

And the same with VOLUME.
Perhaps they have a different meaning depending on the context of use,
but I did not understand this.

2. The second question: if I run several containers, do I understand correctly that they can easily communicate via localhost and do I need to forward some ports for this?

Answer 1

https://docs.docker.com/engine/reference/builder/

The EXPOSEinstruction does not actually publish the port. It functions as a type of documentation between the person who builds the image and the person who runs the container, about which ports are intended to be published.

This is a help to your future container user so that he knows which ports to open. It is not necessary to use.