[CentOS-devel] keeping Extras in sync with Core

So right now following the instructions on:
https://wiki.centos.org/Cloud/Docker
results in a non-working Docker because the
version in Extras hasn't been rebuilt yet, and
there's a strong dependency on selinux policy.

I'm still trying to wade through the pile of
virt7* repos to find something working,
right now hitting a requirement on `skopeo-containers`.

In the future, can we ensure that there's a "CR" version
of Extras[1] that is built at the same time, and they're all released at once?

We also need to figure out how CR interacts with the CBS
content better.

[1] And ideally other things like the atomic host content, but
      let's start with Extras