dockerTools.buildLayeredImage: support fromImage

It is now possible to pass a `fromImage` to `buildLayeredImage` and
`streamLayeredImage`, similar to what `buildImage` currently supports.

This will prepend the layers of the given base image to the resulting
image, while ensuring that at most `maxLayers` are used. It will also
ensure that environment variables from the base image are propagated
to the final image.

doc/builders/images/dockertools.section.md

@@ -111,6 +111,12 @@ Create a Docker image with many of the store paths being on their own layer to i
 
     *Default:* the output path's hash
 
+`fromImage` _optional_
+
+: The repository tarball containing the base image. It must be a valid Docker image, such as one exported by `docker save`.
+
+    *Default:* `null`, which can be seen as equivalent to `FROM scratch` of a `Dockerfile`.
+
 `contents` _optional_
 
 : Top level paths in the container. Either a single derivation, or a list of derivations.