Yes, we do it at Basecamp for all of our legacy apps (and our newest one, HEY), and we've ran Basecamp 2 in production via Kubernetes (we no longer do it, but we did it for a lengthy period of time).
Nothing Kubernetes-related, it was provider-related. We hurriedly moved Basecamp 2 back on-prem after a string of outages caused by our cloud provider at the time. There are a bunch of blog posts on Signal v Noise from early 2019 that talk about it.
Kubernetes for HEY, ECS for the other legacy apps that I mentioned are using Docker, bare metal for Basecamp 2 & 3 (but K8s is on the roadmap for both of them)
GitLab has a pretty nice image for running their Community Edition in Docker.
I've investigated it a few times for several Rails applications that I maintain, but I've never completed a rollout. Running Rails on Docker isn't a huge hurdle, but Capistrano's default git-based deploys are just so _easy and fast_ once the server is set up. I also have to balance deploying Docker with ops' expertise.
I do recommend getting a copy of _Docker for Rails Developers_ to avoid needing to read a bunch of disparate blogs, though it covers Docker Swarm instead of Kubernetes.
We have a number of Dockerized Rails apps hosted on AWS Elastic Beanstalk. It isn't quite as smooth as Heroku but once setup it has been quite solid. The parity between develop and prod is decent, we use docker-compose and localstack to simulate backing services.
We've had our Rails like apps (Sinatra, Padrino) Dockerized for 5 years now and running on container orchestration systems. First Mesos, now Kubernetes. And some running as Lambdas as well. We do not have a single production app not in a container and we are mostly Ruby.
I use docker as a build server. At the end of the project it spits out a debian package that I can install on a machine. It doesn't need bundler, only ruby and the gem command (up to a certain version). Quite a nice setup if you look for bare metal performance
I run my side project in Docker on Kubernetes, works well and wasn’t too annoying to set up. Compared to getting my Go deployments to run it was kind of a pain though.
I've been using Rails for years, I haven't yet experienced much pain by way of language/library versioning (etc). Python is a different story...