chroot as a concept
has an interesting history. There are many web pages describing how, basically, you shouldn't treat it as being a proper security measure, like, ever. I know that things have moved on from naively trusting chroot, and Docker totally isn't just chroot in another guise, but still, I'm not at all surprised that there would be bugs like this in it... (mind you, bugs crop up in vm systems too, from time to time...)
It was probably usability that spurred the development of the first chroot systems, and I'm sure that a similar process took place around the development of containers. Let's hope they can focus on security a bit more to squelch bugs like this so that it doesn't just end up as chroot v2.0.