Docker images used to be, essentially, a linked list of layers. Each
layer would have a tarball and a json document pointing to its parent,
and the image pointed to the top layer:

    imageA  ----> layerA
                    |
                    v
                  layerB
                    |
                    v
                  layerC

The current image spec changed this format to where the Image defined
the order and set of layers:

    imageA  ---> layerA
            |--> layerB
            `--> layerC

For backwards compatibility, docker produces images which follow both
specs: layers point to parents, and images also point to the entire
list:

    imageA  ---> layerA
            |      |
            |      v
            |--> layerB
            |      |
            |      v
            `--> layerC

This is nice for tooling which supported the older version and never
updated to support the newer format.

Our `buildImage` code only supported the old version, so in order for
`buildImage` to properly generate an image based on another image
with `fromImage`, the parent image's layers must fully support the old
mechanism.

This is not a problem in general, but is a problem with
`buildLayeredImage`.

`buildLayeredImage` creates images with newer image spec, because
individual store paths don't have a guaranteed parent layer. Including
a specific parent ID in the layer's json makes the output less likely
to cache hit when published or pulled.

This means until now, `buildLayeredImage` could not be the input to
`buildImage`.

The changes in this PR change `buildImage` to only use the layer's
manifest when locating parent IDs. This does break buildImage on
extremely old Docker images, though I do wonder how many of these
exist.

This work has been sponsored by Target.

dockertools buildImage: support new-style image specs