I hear the "Docker is not a security boundary." mantra all the time, and IIRC it was the official stance of the Docker project a long time ago, but is this really true?

Of course if you have a kernel exploit you'd be able to break out (this is what gvisor mitigates to some extent), nothing seems to really protect against rowhammer/memory timing style attacks (but they don't seem to be commonly used). Beyond this, the main misconfigurations seem to be too wide volume bindings (e.g. something that allows access to the docker control socket from inside the container, or an obviously stupid mount like mounting your root inside the container).

Am I missing something?