Using prepared SQL statements is about writing correct software.

My point is that expecting containers to "secure" broken software is a delusion, especially when the implementation enforcing the constraints--in this case, the Linux kernel--is fundamentally broken from a security perspective in the relevant dimension (that is, local exploits leading to privilege escalation).

Arguing defense in depth in this case is like arguing that stacking 3 web application firewalls on top of an application is better than just 1. The problem being that none of them are capable of addressing the bulk of the proven exploits.

Back to your prepared SQL statement analogy, consider that nobody should be running software with root privileges today, regardless. Whether your network daemon is run in a container or not, it shouldn't being running as root in its steady state. So trying to lock down a container in such an instance is like trying to prevent SQL injection in software that concatenates data into the SQL statement string. It's intrinsically a failed strategy, and not in any sense an engineering solution. It's like saying doing something is better than doing nothing. Well, actually, in reality that's not the case; in reality sometimes doing something is just as bad as doing nothing (sometimes it's even worse, but that's not the argument I'm making here).

There are other, similar reasons why it's a bad strategy to see containers as a security measure. And I fail to see how anybody who has spent any amount of time in this industry, and who has spent any amount of time applying privilege separation principles, could reasonably disagree.

Now, can containers help you write more secure software? Yes, depending on context. But not to the exclusion of other techniques. Containers don't add a useful security layer by themselves, and shipping with slightly tighter capabilities doesn't change this fact in practice. Again, saying that shipping with slightly tighter capabilities (the particular capabilities system at issue here) is better than not is a distinction without a difference when most kernel-based root exploits are in innocuous system calls (e.g. networking system calls) that you could never lock down in a default container setting. If an application writer can't be bothered to make their software work without root, there are evidently other things going on.[1] Again, the problem in reality is software quality--bad kernel quality, bad application quality--and papering over this doesn't help.

[1] A simple, single setuid call shortly after application startup would work at least as well as narrowing process capabilities, though likely be more effective. Even better, write your application to inherit socket and file descriptors so inetd or systemd can implement those privileged operations. These are minimal things all _correctly_ written software should support. If an application doesn't support at least the former, it's fundamentally broken and it's foolhardy to believe this can be papered over. Also, newsflash, systemd doesn't solve PID file races for uncooperative network daemons. That's another instance where people believed somebody came up with some magic software that inherently solved a problem without having to fix the underlying software. The difference being PID file races aren't a comparable security issue because they usually already require local, root capabilities; and systemd's solution arguably makes such a race meaningfully more difficult to exploit. So a misinformed engineer's decision in that context is unlikely to be part of a chain of events leading to billions of dollars of valuation going up in smoke.