better correlation / aggregation of build logs and build status

[tianon]

In the old build system, it was relatively easy to point to something like "here's all the recent builds of the node image, including their build logs" -- with the new system, those are spread out and hard to navigate/find.

For successful builds, we could fix this in an interesting way by making the build logs themselves part of the attestation object that gets pushed to Docker Hub, but it would still be helpful to have a solution for failing builds (and perhaps "status" of builds: queued, in-progress, failing, successful).

See also:

• publish "image source" in the registry #140