Pre steps in docker build with Concourse

Question

I have a concourse pipeline I use to build docker images using this resource. This is a code example:

 - put: build-image.spilo
    params:
      build: git.spilo-dockerfile/
      build_args: {}
      tag_as_latest: true
      tag_file: new-tag-spilo/version
   on_failure:
      params:
        text: 'error message'
      put: slack-alert

For example, I use it to build the spilo zalando docker image. This image comes from the community and I want to use as it is. However, I need to apply to this docker image few patches before run the build and I want to run a pre-build task. This task should run a pre-build.sh script.

The code is more or less like this:

  - task: pre-build-image.spilo
    image: pipeline-image
    input_mapping:
      ...
      repo-code: git.spilo-dockerfile
    output_mapping:
      ???????
    file: ci-cd-pipelines/scripts/pre-build-docker-image.yml
    on_failure:
      params:
        text: 'error message'
      put: slack-alert

My problem is that the pre-build task change the original spilo git project modifying some files but I don't know how to do that. I mean probably I need to use output_mapping but it's not clear to me what to put there and how I can see then the spilo project changes in the next task (the first one above that run the docker build).

Can anyone help?