Fix dependency proxy failures in source change detection templates

Dan Notesteinrequested to merge
fix/dependency-proxy-failures into develop

Summary

  • Replace unreliable CI_DEPENDENCY_PROXY_GROUP_IMAGE_PREFIX/docker:24-cli with the self-hosted docker-builder image
  • The dependency proxy intermittently returns HTML error pages instead of Docker manifests, causing job failures
  • The docker-builder image is hosted in our GitLab registry and is immune to these issues
  • Removed before_script apk add commands since docker-builder already has git/bash

Affected templates

  • .find_upstream_image - used by find_haf_image and similar jobs
  • .check_local_image - used for local image caching

Test plan

  • CI pipeline passes on this branch
  • Test with reputation_tracker pipeline (was failing with this error)

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