Build OCI step runs in the CI_PROJECT_DIR

What does this MR do?

The build OCI step image allows paths to be specified relative to the project dir.

Was:

- name: build
  step: dist://step/oci/build
  inputs:
    ...
    common:
      files:
        /builds/gitlab-org/step-runner/.gitlab/steps/changelog/step.yml: step.yml
    platforms:
      linux/amd64:
        files:
          /builds/gitlab-org/step-runner/changelog-linux-amd64: changelog

Now:

- name: build
  step: dist://step/oci/build
  inputs:
    ...
    common:
      files:
        .gitlab/steps/changelog/step.yml: step.yml
    platforms:
      linux/amd64:
        files:
          changelog-linux-amd64: changelog

This MR also promotes the changelog step to demonstrate working OCI image behaviour. The release step will be updated to use the changelog step image once a Runner is released with step-runner version 0.13.0.

Reference

• Dogfood Step images in Steps Pipeline (#234)