Support :custom_dns when starting a container

To start a container with a custom DNS server address, (that is,
analogous to 'docker run --dns=8.8.8.8'), do
"set :custom_dns, '8.8.8.8'".
Document :custom_dns in README
Merge pull request #63 from bryannewrelic/bryan/add_dns_setting

Support :custom_dns when starting a container