Do they ever offer to run your container in the same VM as those of other customers?

They never do this. For secure isolation, they only trust VM isolation. It seems unlikely that this will change.

> What would even be the point of user namespacing, network namespaces, filesystem namespaces, etc... if not security?

They're for installation/configuration/administration. They allow you to run multiple applications on one Linux VM, and to configure them independently, almost as if you were running multiple VMs (with the advantage of lower overheads - only one instance of the kernel).

Kubernetes puts this to good use, letting you treat application deployments as commodities across your cluster.

Containers do not offer secure isolation. They are by nature much leakier than the isolation VMs can offer. The Docker folks still treat isolation-failures as bugs, of course. (Well, ignoring things like the way 'uptime' gives the uptime of the underlying machine, and not of your container.)