Update container networking page

[bermudezmt]

Problem description

docker/for-linux#488

So, looks like that part of the docs need some updating;

explain that when systemd-resolvd is active, the /run/systemd/resolve/resolv.conf file is used instead of /etc/resolv.conf
I think the mention of /etc/hosts should be removed there; I don't think it's used, other than when running with --network=host (but perhaps I'm overlooking something)
explain that (unless a --dns is set on docker run, or configured in the daemon defaults), docker will copy the host's DNS settings to the container when it's created

Problem location

https://docs.docker.com/config/containers/container-networking/#dns-services

Affected versions: Engine 18.09 and above

Suggestions for a fix

Follow discussion and translate docker/for-linux#488 (comment) into product docs. @thaJeztah my lazy issue tracking. Thanks again for all that you do. 😇

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