The issue here with containers in general is that they break well known security boundaries. This is exacerbated by things like k8s cause it is inherently built to orchestrate many of them and assumes that there is a multi-tenant deployment by default.

Most recently Google themselves wrote about this just a few weeks ago:

https://cloud.google.com/blog/products/containers-kubernetes...

but the issues have been present forever:

https://capsule8.com/blog/oomypod-nothin-to-cri-o-bout/

https://www.zdnet.com/article/doomsday-docker-security-hole-...

https://www.zdnet.com/article/alpine-linux-docker-images-shi...