Publish multi-arch helper images

Description

Once #25487 (closed) is implemented, we'll have helper images being built with docker buildx (with multi-platform support), but still publishing to the same platform-dependent tags. The goal of this issue is to migrate to a new tag structure which doesn't include the platform.

Older Proposal

1. Start publishing multi-arch images and phase out the old tagging structure. This will take time and we have to update both Runner code and build process to use it.

Acceptance Criteria

• Per-OS Multi-Arch Manifests
  • v18.9.0-alpine (all Linux arches)
  • v18.9.0-ubuntu (all Linux arches)
  • Windows per-version manifests (all arches)
• The "Special One" - Cross-OS Manifest
  • v18.9.0 or latest → Contains Alpine (all arches) + Windows (all versions/arches)
• Older tags and arch specific images should still be available because runner still depends on it.

Links to related issues and merge requests / references

#25487 (closed)