Prune images after successful Kubernetes upgrade

[judge-red]

Description of the feature you would like to add / User story

After ~2 Kubernetes version upgrades, we always receive alerts as disk usage has climbed >80% (of 50GB). We then address this by SSHing into all control plane nodes and executing sudo crictl rmi --prune to delete unused images, bringing us down to ~40% usage.

It would be nice if we could tell kubeone, probably with a flag, to prune the images on all control plane nodes after the control plane has been upgraded. It's not super elaborate to do it by hand, but it would be a quality of life improvement. The actual cleanup isn't even the "hard" task, but the alert that fires first is often something unrelated (e.g. node-exporter running out of disk space) and thus it first takes a bit of investigation.

With a --prune-images flag, we'd just add that to our pipeline job and wouldn't have to care about it anymore.

Solution details

Add a flag, such as --prune-images, to the kubeone CLI that runs sudo crictl rmi --prune on each control plane node after the upgrade (either on that node or of the whole cluster - you're probably the better judge of that). I guess ideally it could also be used to prune images even when no upgrade is performed, in case people want to run it separately after the upgrade was done.

Alternative approaches

SSH into each control plane node and execute the command manually.

Use cases

Additional information

[csengerszabo]

/assign @mohamed-rafraf