ci: add Dockerfile for Railway compile-test

Multi-stage build verifying default + jit-native features and HPC tests.

https://claude.ai/code/session_01CdqyUTUfjKZuk8YGJzv6LB

Author: claude

.dockerignore

@@ -0,0 +1,9 @@
+target/
+.git/
+.claude/
+*.md
+!README*.md
+.github/
+benches/
+examples/
+scripts/

Dockerfile

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+# ndarray — Railway compile-test image
+# Verifies the HPC module builds cleanly (default + jit-native features)
+#
+# Build: docker build -t ndarray-test .
+# Run:   docker run --rm ndarray-test
+
+FROM rust:1.85-slim AS builder
+
+WORKDIR /app
+
+# System deps for Cranelift JIT
+RUN apt-get update && apt-get install -y --no-install-recommends \
+    pkg-config libssl-dev \
+    && rm -rf /var/lib/apt/lists/*
+
+# Copy workspace files first for layer caching
+COPY Cargo.toml Cargo.lock ./
+COPY ndarray-rand/Cargo.toml ndarray-rand/Cargo.toml
+COPY crates/ crates/
+
+# Copy source
+COPY src/ src/
+COPY ndarray-rand/src/ ndarray-rand/src/
+
+# Build default features
+RUN cargo build --release 2>&1 && echo "=== DEFAULT BUILD OK ==="
+
+# Build with JIT
+RUN cargo build --release --features jit-native 2>&1 && echo "=== JIT-NATIVE BUILD OK ==="
+
+# Run tests
+RUN cargo test --release --lib -- hpc:: 2>&1 && echo "=== HPC TESTS OK ==="
+
+# Minimal runtime image — just proves it compiled
+FROM debian:bookworm-slim
+COPY --from=builder /app/target/release/libndarray.rlib /usr/local/lib/
+CMD ["echo", "ndarray build verified"]