chore(docker): improve Docker build caching

[msukkari]

Summary

• enable Dockerfile frontend 1.7 to support BuildKit cache mounts
• add persistent BuildKit cache mounts for Go module/build caches in Zoekt build steps
• add persistent BuildKit cache mounts for Yarn cache during workspace installs in Docker build stages
• use sharing=locked on shared Yarn cache mounts to avoid concurrent write races in parallel build stages
• enable GitHub Actions cache import/export in pr-gate docker builds so PR checks benefit from the same layer cache mechanism as release builds
• add an [Unreleased] CHANGELOG.md entry for this PR

Validation

• cold build benchmark before/after and image size comparison captured in:
  • docker_build_benchmark_summary.txt
• container smoke test performed against optimized image with a valid config file:
  • container starts, migrations apply, backend/web/zoekt/redis all report running
  • curl -sSL -o /tmp/sourcebot-app-body.txt -w '%{http_code}' http://localhost:3900/~ => 200

Results

• baseline cold build: 259.552s
• optimized cold build: 231.894s (about 10.7% faster)
• optimized warm build: 1.196s (full cache hit)
• baseline image size: 1890947560 bytes
• optimized image size: 1890947576 bytes (delta +16 bytes, effectively unchanged)

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