docker: lowercase image name and tag

The docker loading (docker 1.12.6) of an image with uppercase in the
name fails with the following message:
invalid reference format: repository name must be lowercase

pkgs/build-support/docker/default.nix

@@ -409,8 +409,9 @@ rec {
         };
       result = runCommand "docker-image-${baseName}.tar.gz" {
         buildInputs = [ jshon pigz coreutils findutils ];
-        imageName = name;
-        imageTag = tag;
+        # Image name and tag must be lowercase
+        imageName = lib.toLower name;
+        imageTag = lib.toLower tag;
         inherit fromImage baseJson;
         layerClosure = writeReferencesToFile layer;
         passthru.buildArgs = args;