How to access folder from another site?

I don’t understand much about this new docker system used in EE4. I have a problem I have a php script that is in /opt/easyengine/sites/ with real path in

I would like to access a file that is in /opt/easyengine/sites/ /var/lib/ docker/ volumes/site2com_htdocs /_data/htdocs/public

This is making me very confused, I don’t know how I do it with docker

In a normal configuration it was just using the relative path that worked. But with docker I can’t do that.