This just had the wrong logic so it was outputing the container rootfs
to the filesystem instead of to the image store.

Signed-off-by: Brian Goff <[email protected]>

Before this was just using whatever created time was set on the base
image config which was incorrect.

Example using the `docker buildx bake runc-jammy-container` command:

Before:

docker image ls | grep runc
runc                                          jammy             8d1d42e074f3   25 minutes ago   228MB

After:

  docker image ls | grep runc
  runc                                          jammy             03c24620c8fa   5 seconds ago    228MB

**Note**: how the created time before was 25mins ago (b/c apparently the base
jammy image was recently updated, I've seen this be 2 weeks) compared to
after its 5 seconds ago.

Signed-off-by: Brian Goff <[email protected]>