In case you missed it, today CoreOS introduced its own container format called Rocket (to compete with Docker). It's hard to decide who actually knows what they're talking about here. The CoreOS folks have proven they have good judgement when it comes to reimplementing existing infrastructure. Docker's CEO posted a rebuttal that's encouraging, but it rings hollow to me.
Separately, Vagrant is barely mentioned these days. The lines between all three options (Docker, Rocket, Vagrant) blur for most people. For example, check out this Stack Overflow thread from about a year ago where the creator of Docker and the creator of Vagrant go head-to-head on the question, "Should I use Vagrant or Docker?" They're both a bit confused, but Docker has captured mindshare since then.
My take on this is things are moving so fast nobody knows where it'll end up. Docker was first demoed at PyCon in March 2013 — so recently! Yet when you see both Amazon and Google announcing Docker container services within weeks of each other, it's easy to assume that Docker has won. The beauty of open source and high-level APIs is that it's still anyone's game. Confident broadsides like Rocket cast doubt on our assumptions. I applaud CoreOS for keeping everyone alert.
Meanwhile, Solomon Hykes (Docker's creator and CTO) still knows what's up. Here's his Tweet stack on the matter. His words ring true to me. Why isn't Docker responding on their blog with strong technical leadership like that? After Twitter and others destroyed their ecosystems, we're all a bit skeptical of rhetoric about "open" these days.