[drothlis]

I realised this is another example of "trees" as first-class citizens in a build system. In my comment above the tree we're passing around is a docker layer; in my LWN article it's an OSTree ref. We use the former for our cloud containers, the latter for our embedded device rootfs and systemd-nspawn containers.

I suppose we could use systemd-nspawn on our cloud servers too, instead of docker, but when we wrote the build system we were already using docker so it was the expedient thing to do at the time.