I think you cannot.

I'm familiar with Cloud Run but I'm not familiar with rails.

I assume you'd need to be able to shell into a container in order to be able to run IRB. Generally, you'd do this by asking the runtime (Docker Engine, Kubernetes, Cloud Run) to connect you to the container so that you could do this.

Cloud Run does not (appear) to permit this. I think it's a potentially useful feature request for the service. For those containers that contain shells, this would be the equivalent of GCE's gcloud compute ssh.

Importantly, your app may be serviced by multiple, load-balanced containers and so you'd want to be able to console into any of these.

However, you may wish to consider alternatives mechanisms for managing your app: monitoring, logging, trace etc. These mechanisms should provide you with sufficient insight into your app's state. Errant container instances should be terminated.

This follows the concept of "pets vs. cattle" whereby, instead of nurturing individual containers (is one failing?), you nurture the containers holistically (is the service comprising many containers failing?)

For completeness, if you think that there's an issue with a container image that you're unable to resolve through other means, you could run the image elsewhere (e.g. locally) where you can use IRB. Since the same container image will behave consistently wherever it's run, you should be able to observe the issue using IRB locally too.