Stop copying bin directory files into container image

This merge request updates the CI/CD pipeline configuration and Docker setup. The main changes are:

CI config improvements: The build trigger logic was modernized from the older "only" syntax to the newer "rules" syntax, which is more flexible and reliable.

Dockerfile cleanup: Stop copying all project files into the Docker container, because the project gets cloned into the container by the CI runner anyway. So we will still have access to the project files in the CI job. The benefit is that we don't need to rebuild the CI image every time a script in bin directory is updated. I temporarily added a job to the pipeline to show that the files are present: https://gitlab.com/gitlab-org/merge-train/-/jobs/11455040671.

How to rollback this change after merging?

• Revert this MR
• If the new image built after the revert is merged has issues, update the merge:security and merge:foss CI jobs to use "${CI_REGISTRY_IMAGE}:001f977c" as the image.

Related to gitlab-com/gl-infra/delivery#21562