update warning about local dns resolvers

This changes the local DNS resolver warning to explain what the
warning is about and why the default external DNS server is used.

Docker-DCO-1.1-Signed-off-by: Cristian Staretu <cristian.staretu@gmail.com> (github: unclejack)
Merge pull request #4188 from unclejack/update_dns_resolver_warning

update warning about local dns resolvers