docs: explain DNS warnings & how to fix them

This explains how to fix the DNS warnings on Ubuntu and why they're
shown.

Docker-DCO-1.1-Signed-off-by: Cristian Staretu <cristian.staretu@gmail.com> (github: unclejack)
Merge pull request #4265 from unclejack/explain_networkmanager_and_dns_problems

docs: explain DNS warnings & how to fix them