tracker: bootc container storage

[cgwalters]

As part of both logically bound images as well as migrating to a containers-image/storage backend we should actually as a preparatory step introduce the "bootc container storage".

Current architecture

Looking at a physical bootc-installed filesystem basically all content is underneath /ostree, and the content managed by bootc itself is mostly under /ostree/repo. I think we should introduce /ostree/bootc as its own dedicated directory.

Support usage from podman

We could have a symlink inside the booted root as /usr/lib/bootc/storage-root or so that would point to /sysroot/ostree/bootc. Then, users of logically bound images would either need to explicitly configure that as --root, or possibly we encourage configuring the podman stack to use this as an additional store.