This new entrypoint takes an Ignition config and applies it to the
current rootfs (or one provided by `-root`). This tool could then be
used as part of build processes where we want to bake an Ignition
config, such as in CoreOS layering:
https://github.com/coreos/enhancements/blob/main/os/coreos-layering.md
This in turn should also help drain from the MCO the re-implementation
of large parts of the Ignition spec.
Another approach would be to build on top of `ignition -stage=files`,
but in the end I think we want a cleaner dedicated UX for this workflow
long-term (and in fact we should clean up the hacky places where we call
Ignition like this).
It shouldn't be used by end-users to live apply changes to their
Ignition config. A check that we are in a container is added as a
safeguard against this.
By default, unsupported modifications (like partitioning) trigger
errors, though one may pass `-ignore-unsupported` to override this. Also
by default, remote resources are automatically fetched, though one may
pass `-offline` to override this.
