It's just busybox with the command already set to sleep infinity. You can change an image for a container in a running pod, but you can't change its command or args, so in cases where those are unset, this provides a way to troubleshoot the container's environment.

-image: "your-app"
+image: "ghcr.io/mlibrary/kube-debug"

It builds nightly based on busybox:latest. In order to get this to work, I had to go to Settings > Actions > General and set Workflow permissions to Read and write.