We cannot recommend one or couple specific tools because they always change. At the moment of writing there are couple of market leaders as Jenkins and GitlabCI, but there are many others that can fulfil your needs in CI/CD system. With evolution of Kubernetes there are more and more companies who try to adopt the GitOps approach and new tools are emerging , such as Jenkins X, ArgoCD, Flux, Spinnaker. The main criteria you should look for is how easy it is to customize the CI/CD tool you will use, and how much of that customization you will need. Everything else is very specific to the company and product you would like to build, for some people simple TravisCI may be enough to build the ruby code and deploy it somewhere, and for other company building 100 microservices - you would need tool like Jenkins with ability to add your own custom logic into the CI/CD process.