Dockerfiles restructuring

What does this MR do?

This MR:

• renames dockerfiles/build (a generic name which doesn't mention anything about helper images) to dockerfiles/runner-helper.
• moves dockerfiles/alpine and dockerfiles/ubuntu under a new dockerfiles/runner/ directory.
• changes scripts to build the helper binaries under out/binaries (to be consistent with the Runner build process), instead of building it under dockerfiles/runner-helper/binaries and then copying it out to out/binaries/gitlab-runner-helper/

Why was this MR needed?

I feel that the existing folder structure is not obvious to people who are onboarding or external contributors, so by making things more obvious we'll be lowering a bit the overhead required to work with the codebase.

Are there points in the code the reviewer needs to double check?

Does this MR meet the acceptance criteria?

• Documentation created/updated
• Added tests for this feature/bug
• In case of conflicts with master - branch was rebased

What are the relevant issue numbers?

Closes #25846 (closed)