Support for host storage driver "vfs" (and performance implications of "overlay")

[JustBeefalo]

Due to an observant CPU bottleneck slowing our disk I/O when using the "overlay" storage driver, we wanted to test if we could get speed ups using the "vfs" driver. However, bootc-image-builder fails when we change our storage driver.

E.g.,

# /etc/containers/storage.conf
[storage]
driver = "vfs"

$ sudo podman run --rm -ti --privileged --security-opt label=type:unconfined_t \
    -v "${PWD}/output:/output" \
    -v "/etc/containers/storage.conf:/etc/containers/storage.conf" \
    -v "/var/lib/containers/storage:/var/lib/containers/storage" \
    localhost/bootc-image-builder --local --type qcow2 --chown "$(id -u):$(id -g)" "${IMAGE_NAME}:${IMAGE_TAG}"
WARNING: --local is now the default behavior, you can remove it from the command line
[|] Manifest generation step
Message: Generating manifest manifest-qcow2.json
2025/07/14 20:24:41 error: cannot build manifest: cannot resolve containers: failed to resolve container: '': error getting manifest: reference "[overlay@/var/lib/containers/storage+/run/containers/storage]<image-name:tag>" does not resolve to an image ID

${IMAGE_NAME}:${IMAGE_TAG} definitely being a valid image I've already pulled.

Looks like it might be the same as (or related to) #327, however I can only reproduce the error when using the "vfs" storage driver. "overlay" works fine, our build times are just too long.

Using bootc-image-builder 9.6.