You'll see a maturity in this space after the Open Containers Initiative standardizes the container image format. Then, developers can focus on UX improvements rather than worrying about what the build tool should even be producing.

Reliance on initiatives and standards bodies to provide a guidance for maturity, at least in recent years, is a fools errand. They usually end up rubber stamping what's been the norm and work back from there.

That's true, OCF is more or less the same as the Docker container format.

If you are looking at rkt, if you have time, take a look at Kurma (open-source too). Kurma was built using the same specification rkt was built from. Here is the getting started guide: http://kurma.io/documentation/kurmad-quick-start/ What I like of kurma is that I can simply run docker images from the hub, no need to download, convert, etc.

Disclaimer: I am an Apcera employee and Kurma is an open-source project sponsored by Apcera