optimize: Dockerfile with Rust dependency caching

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

infra/colony/Dockerfile

@@ -6,11 +6,22 @@ RUN npm ci
 COPY ui/colony/ .
 RUN npm run build
 
-# Stage 2: Build backend
+# Stage 2: Build backend (with dependency caching)
 FROM rust:1.86-slim AS backend
 RUN apt-get update && apt-get install -y pkg-config libssl-dev && rm -rf /var/lib/apt/lists/*
 WORKDIR /app
+
+# Cache dependency build: copy manifests first, create stub sources, build deps
 COPY Cargo.toml Cargo.lock ./
+COPY crates/colony-types/Cargo.toml crates/colony-types/Cargo.toml
+COPY crates/colony/Cargo.toml crates/colony/Cargo.toml
+RUN mkdir -p crates/colony-types/src crates/colony/src \
+    && echo "fn main() {}" > crates/colony/src/main.rs \
+    && echo "" > crates/colony-types/src/lib.rs \
+    && cargo build --release -p colony 2>/dev/null || true \
+    && rm -rf crates/
+
+# Now copy real source and build (deps already cached)
 COPY crates/ crates/
 RUN cargo build --release -p colony