dockerTools.pullImage: expose image* attributes (#41366)

Attributes `imageName` and `imageTag` are exposed if the image is
built by our Nix tools but not if the image is pulled. So, we expose
these attributes for convenience and homogeneity.

pkgs/build-support/docker/default.nix

@@ -47,6 +47,8 @@ rec {
     }:
 
     runCommand name {
+      inherit imageName imageDigest;
+      imageTag = finalImageTag;
       impureEnvVars = pkgs.stdenv.lib.fetchers.proxyImpureEnvVars;
       outputHashMode = "flat";
       outputHashAlgo = "sha256";