Apply Phase D playbook from docker-build-optimization-roadmap.md:
- scripts/docker-prep.sh: synced from canonical template
(learning_ai_common_plat@a418a23e). Adds --dry-run, --check,
--strip-overrides, idempotency guard, trap-based auto-restore,
portable sed, .gitkeep preservation.
- scripts/_docker-prep-inject.js: helper for pnpm.overrides injection
- scripts/_docker-prep-strip.js: helper for --strip-overrides mode
- scripts/docker-doctor.sh: thin wrapper to canonical linter
- Makefile: make doctor target (gitea-doctor + docker-doctor)
Per-repo Dockerfile/compose fixes flagged by docker-doctor are tracked
in the roadmap and will land in follow-up commits.
Refs: docker-build-optimization-roadmap.md §Phase D
