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'".
Merge pull request #65 from didip/master

deploy:dogestry:pull_image is now a 2 steps process.
Document :custom_dns in README
Merge pull request #63 from bryannewrelic/bryan/add_dns_setting

Support :custom_dns when starting a container
Per Excon#357 this is the correct solution to not include port in Host.
Fix the 1 broken test.