Remove dependency on docker import

@adamjt @bfulton @tlunter 

This pull adds `Dockly::Docker#registry_import`, which allows the user to specify a registry to build from instead of a url/s3path.

Examples:
```ruby
docker :my_docker
   ...
   registry_import 'nahiluhmot/base'
end


docker :other_docker
  ...
  registry_import 'custom.registry.com/my-image'
end
```