Using Kubernetes with ruby on rails application
How will you use nginx?
A kubernetes service can be backed by several kubernetes pods. Whenever anyone makes a request to the kubernetes service, the request is sent to one of the upstream pods in a round robin fashion.
If you were planning to use nginx as a 'load-balancer' or reverse proxy to front your rails app, you don't really need that anymore. Each pod ofcourse will need to have something like passenger/unicorn to serve the rails app.
Here'a a guide I found that talks about a rails deployment from start to end: http://www.thagomizer.com/blog/2015/07/01/kubernetes-and-deploying-to-google-container-engine.html
If you're planning to use nginx as a static file server, my recommendation would be to have a different pod for the static files that just contains nginx.
What is better to use with k8s?
K8s doesn't really care, because this is outside k8s's concern. Use whatever you like, or whatever you think works better in a container environment. The better question to ask might be which one of passenger/unicorn is a better fit for containerised rails apps.