Support switching to another bootable container image before applying configuration

[travier]

Feature Request

Environment

N/A

Desired Feature

With bootable containers, users will be able to customize their operating system using a container build process. This feature would allow them to boot a pre-existing system image (for example a regular Fedora CoreOS image) and then switch to their image while in the initramfs, before applying the rest of the Ignition configuration.

Other Information

See: https://gitlab.com/fedora/bootc/tracker/-/issues/53