Expose option to set target-imgref to a different value from the source imgref

[achilleas-k]

I can imagine a few use cases where one might want to build a disk image from a source but set the target image ref to something different. For example, build pipelines with reproducible configurations that pull a specific image version, or an image from an internal container registry, but need the final image to have an update target that pulls from a floating tag or public registry.

This feature would enable something like:

› sudo podman run \
    --rm \
    -it \
    --privileged \
    --pull=newer \
    --security-opt label=type:unconfined_t \
    -v ./output:/output \
    -v /var/lib/containers/storage:/var/lib/containers/storage \
    -v ./config.toml:/config.toml \
    quay.io/centos-bootc/bootc-image-builder:latest \
    --target-imgref registry.example.com/container:<floating tag> \
    registry.example.com/container:<some specific version>

which will flow down to the bootc install command to use <some specific version> to build the image but set the target-imgref to <floating tag>

bootc install to-filesystem ... --target-imgref registry.example.com/container:<floating tag>

Right now, the --target-imgref is always set to the container source value from the bootc-image-builder call registry.example.com/container:<some specific version>.

This should also work for the ISO builds where we set the target-imgref using a %post section in the kickstart using bootc switch --mutage-in-place ....