streamLayeredImage: pass through passthru attributes

This is useful for a use-case we have with a Nix-based CI system that
specifies things like deploy steps as passthru attributes[0].

Previously the only way to do this would have been to concatenate
attributes onto the resulting derivation, but passing them in and
actually treating them as proper passthru attributes is cleaner.

[0]: https://cs.tvl.fyi/depot@f7d7da6aceb407b719cf4683a75878fd3aca319e/-/blob/nix/buildkite/default.nix?L222-226

pkgs/build-support/docker/default.nix

@@ -827,6 +827,8 @@ rec {
       # this on, but tooling may disable this to insert the store paths more
       # efficiently via other means, such as bind mounting the host store.
       includeStorePaths ? true
+    , # Passthru arguments for the underlying derivation.
+      passthru ? {}
     ,
     }:
       assert
@@ -987,7 +989,7 @@ rec {
         result = runCommand "stream-${baseName}"
           {
             inherit (conf) imageName;
-            passthru = {
+            passthru = passthru // {
               inherit (conf) imageTag;
 
               # Distinguish tarballs and exes at the Nix level so functions that