I understand that the focus/brand of Docker is aimed at a slightly different scenario.

What I'm not seeing is why -- from an objective, technical point of view -- you couldn't do almost any of the same things with plain LXC these days, perhaps with LXD on top if the vanilla UI for setting things up isn't sufficient.

I mean, Docker Hub and some nice UI tools are great and all, but I don't see a USP or a defensible position worth a billion dollar valuation in there. So what is really behind the confidence that investors at this level must surely have?

Yes, you can do this with most other container systems. But people haven't done this, except with Docker. Docker has a significant lead on competitors. People have little reason to adopt a competitor as a result.

OK, but presumably most of those people also aren't paying anything to Docker-the-business for using it right now. If they want to be worth billions, there will have to be real revenues sooner or later, and at that point whoever is paying does have an incentive to look at competitors. So at the risk of repeating myself, I'm still wondering what they have as a USP or other effective barrier to competitors stealing away their market share as fast as they built it up. Surely there must be something if investors are actually going in at this level, and I'm very curious to know what it might be.

Docker breaking shit all the time is probably a reason to adopt a competitor.