Both Dockerfiles already install mold v2.35.1 but the registry images
are stale builds without it. This change triggers rebuild-ci-builder
and rebuild-ci-builder-cpu pipeline steps via detect-changes.
.cargo/config.toml uses -fuse-ld=mold — without mold in the image,
linking falls back to the system default (slower).
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Based on rust:1.89-slim-bookworm (~2-3GB vs ~8GB CUDA devel).
Same toolchain: mold 2.35, protoc 28.3, sccache 0.10, clang, lld.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>