Add Rust port of the finger daemon

Tokio-based reimplementation in rust/, mirroring the C++ handler and
ban-tracker logic (directory-traversal checks, case-insensitive plan
lookup, rolling-window IP ban tracking, allowlist parsing) along with
its full test suite. Includes a matching multi-stage Dockerfile.

rust/Dockerfile

@@ -0,0 +1,42 @@
+# Multi-stage build for the Rust finger service
+# Build stage
+FROM rust:slim AS builder
+
+WORKDIR /app
+COPY . .
+
+RUN cargo build --release
+RUN cargo test --release
+
+# Runtime stage — match the C++ image's base so OS-level overhead (syscalls,
+# libc) is comparable between the two for benchmarking.
+FROM ubuntu:24.04
+
+ENV DEBIAN_FRONTEND=noninteractive
+
+RUN apt-get update && apt-get install -y --no-install-recommends \
+    netcat-openbsd \
+    && rm -rf /var/lib/apt/lists/* \
+    && (userdel -r ubuntu 2>/dev/null || true) \
+    && groupadd -g 1000 finger \
+    && useradd -m -u 1000 -g finger -s /bin/sh finger
+
+COPY --from=builder /app/target/release/finger /usr/local/bin/finger
+RUN chmod +x /usr/local/bin/finger
+
+RUN mkdir -p /var/finger/users && \
+    chown -R finger:finger /var/finger
+
+USER finger
+
+EXPOSE 79
+
+HEALTHCHECK --interval=30s --timeout=3s --start-period=5s --retries=3 \
+    CMD nc -w 1 127.0.0.1 79 < /dev/null || exit 1
+
+LABEL org.opencontainers.image.title="finger"
+LABEL org.opencontainers.image.description="A silly finger service written in Rust"
+LABEL org.opencontainers.image.source="https://github.com/waffle2k/finger"
+LABEL org.opencontainers.image.licenses="MIT"
+
+CMD ["finger"]