dockerTools: add streamed image as passthru to buildLayeredImage

This is convenient for debugging the underlying streamed image used by
`dockerTools.buildLayeredImage`.

Here's an example of how you might use this:

```console
$ nix repl ./.
nix-repl> dockerTools.examples.nginx.passthru.stream
«derivation /nix/store/9zczmlp2kraszx4ssmh6fawnlnsa5a4n-stream-nginx-container.drv»
```

pkgs/build-support/docker/default.nix

@@ -523,7 +523,7 @@ rec {
     runCommand "${baseNameOf name}.tar${compress.ext}"
       {
         inherit (stream) imageName;
-        passthru = { inherit (stream) imageTag; };
+        passthru = { inherit (stream) imageTag; inherit stream; };
         nativeBuildInputs = compress.nativeInputs;
       } "${stream} | ${compress.compress} > $out"
   );