What determines if code is "untrusted" or not anyway? It's fine to run postgres or redis (someone else's code! that I have certainly not audited!) on a server, but as soon as you run it in a container that's... less secure?

That is certainly the perception. Like any software, you have to make sure you're pulling from a reliable source--if you're pulling an image from `hub.docker.com/r/definitely-not-a-hacker/postgres` rather than the official postgres image, you're exposing yourself. But it's transparently ignorant to argue that this is particular to containers--one can also download a postgres ELF binary from an untrusted source.

I really think a lot of criticism of containers is absurdly low quality (e.g., criticizing containers for issues that are universal to all software)--it feels like people are really grasping at straws. One gets the distinct impression that some people have spent years or even decades perfecting bespoke, rube-goldberg-esque application runtime environments and now containers are obsoleting their value proposition. Of course, I'm very hesitant to psychoanalyze and would never argue that any individual is so motivated, but this is the impression I get in aggregate.

Surely it's a bit more complex question than that. The traditional way of running software includes some sort of privilege management, uids, ulimits, chroots but sometimes also things like pledge and selinux. Those things are sometimes summarizes as privilege minimization.

Privilege minization is much harder when stuffing everything in a container. I'd wager that running Chrome normally is probably safer than running it inside Docker, for example, because not all sandboxing functionality works when running inside a container.

So it would depend on what software, and what type of container.

I suspect the overwhelming majority of software shops aren't doing the diligence you describe as "traditional". For those folks, containers represent a strict improvement in security. I would be curious to learn more about which "privilege minimization" features are incompatible with containers, however.