Docker PHP interpreter doesn't work via docker-compose

PHPStorm 2018.1

docker-compose version 1.8.0, build unknown

Ubuntu 16.04

Docker version 1.13.1, build 092cba3

I have a project for docker-compose configs. The fpm section looks like this

The projects works pretty good.

PHP scripts run nicely via containers' bash sessions

I tried to be able to run scripts via Phpstorm GUI, you know:

Firstly I created a new interpreter setting

Path mappings looks fine 

At Server section I set up mappings too. I'm not sure it is really necessary:

Now, we are ready to test the remote interpreter!

And... it fails:

Trying to do it several times I noticed every time a new container is being created. Why?

May be this is the reason of failing? 

Why does phpstorm ignores a running container?

When I use Docker container (not compose) as remote interpreter, all works fine:

By the way: what is the string in Path Mappings means? What is /opt/project? I never set it, but it works (even xdebug)