feat(ci): add CUDA-free CI builder image for service compilation

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>

infra/docker/Dockerfile.ci-builder-cpu

@@ -0,0 +1,47 @@
+# CUDA-free CI builder image for Foxhunt service compilation
+# ~2-3GB vs ~8GB for the CUDA devel variant (Dockerfile.ci-builder)
+# Contains: Rust 1.89, protoc, sccache, git, OpenSSL, clang, mold, lld, make
+# Build: docker build -f infra/docker/Dockerfile.ci-builder-cpu -t foxhunt-ci-builder-cpu .
+# Push:  docker tag foxhunt-ci-builder-cpu rg.fr-par.scw.cloud/foxhunt-ci/ci-builder-cpu:latest
+#        docker push rg.fr-par.scw.cloud/foxhunt-ci/ci-builder-cpu:latest
+
+FROM rust:1.89-slim-bookworm
+
+ENV DEBIAN_FRONTEND=noninteractive
+
+RUN apt-get update && apt-get install -y --no-install-recommends \
+    git \
+    curl \
+    ca-certificates \
+    pkg-config \
+    libssl-dev \
+    lld \
+    clang \
+    make \
+    perl \
+    build-essential \
+    libfontconfig1-dev \
+    unzip \
+    && rm -rf /var/lib/apt/lists/*
+
+# mold linker — 2-5x faster than lld for final link step on large Rust binaries
+RUN curl -fsSL https://github.com/rui314/mold/releases/download/v2.35.1/mold-2.35.1-x86_64-linux.tar.gz \
+    | tar xz -C /usr/local --strip-components=1 \
+    && chmod +x /usr/local/bin/mold
+
+# protoc 28.3 (proto3 optional support, matches local dev)
+RUN curl -fsSL https://github.com/protocolbuffers/protobuf/releases/download/v28.3/protoc-28.3-linux-x86_64.zip -o /tmp/protoc.zip \
+    && unzip -o /tmp/protoc.zip -d /usr/local bin/protoc 'include/*' \
+    && rm /tmp/protoc.zip \
+    && chmod +x /usr/local/bin/protoc
+
+# sccache for S3-backed build caching
+RUN curl -fsSL https://github.com/mozilla/sccache/releases/download/v0.10.0/sccache-v0.10.0-x86_64-unknown-linux-musl.tar.gz \
+    | tar xz -C /usr/local/bin --strip-components=1 sccache-v0.10.0-x86_64-unknown-linux-musl/sccache \
+    && chmod +x /usr/local/bin/sccache
+
+# clippy pre-installed
+RUN rustup component add clippy
+
+# Verify
+RUN rustc --version && cargo --version && git --version && protoc --version && sccache --version && make --version && mold --version