dockerTools: Fix passthru image tag

It should match the actual image tag.
This fixes the problem introduced in 00996b5e03
https://github.com/NixOS/nixpkgs/pull/115491#pullrequestreview-672789901

pkgs/build-support/docker/default.nix

@@ -536,7 +536,7 @@ rec {
         passthru.layer = layer;
         passthru.imageTag =
           if tag != null
-            then lib.toLower tag
+            then tag
             else
               lib.head (lib.strings.splitString "-" (baseNameOf result.outPath));
         # Docker can't be made to run darwin binaries