Add multiple layers to mitigate dependency issues.

rust-engine/Dockerfile

@@ -2,15 +2,31 @@
 
 # --- Stage 1: Builder ---
 # Use a stable Rust version
-FROM rust:stable-bookworm AS builder
+FROM rust:1.82-slim AS builder
 WORKDIR /usr/src/app
 
 # Install build dependencies needed for sqlx
 RUN apt-get update && apt-get install -y --no-install-recommends \
     pkg-config \
     libssl-dev \
+    curl \
+    build-essential \
+    ca-certificates \
     && rm -rf /var/lib/apt/lists/*
 
+# Allow override of toolchain (stable, nightly, or a pinned version)
+ARG RUSTUP_TOOLCHAIN=stable
+
+# Explicit HOME to avoid undefined variable during Docker build
+ENV HOME=/root
+
+# Install rustup and ensure the selected toolchain is installed (fixes edition mismatches in CI)
+RUN curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y \
+    && /root/.cargo/bin/rustup toolchain install ${RUSTUP_TOOLCHAIN} \
+    && /root/.cargo/bin/rustup default ${RUSTUP_TOOLCHAIN}
+
+ENV PATH="/root/.cargo/bin:${PATH}"
+
 # Copy manifest files first to leverage Docker layer caching for dependencies
 COPY Cargo.toml Cargo.lock ./