reuse previous build artifacts

backend/Dockerfile

@@ -2,10 +2,14 @@
 FROM rust:1.94-bookworm AS builder
 
 WORKDIR /usr/src/app
-COPY Cargo.toml Cargo.lock* ./
-COPY src/ src/
 
-RUN cargo build --release
+# Cache dependency build: only re-runs when Cargo.toml/lock change
+COPY Cargo.toml Cargo.lock* ./
+RUN mkdir src && echo 'fn main() {}' > src/main.rs && cargo build --release && rm -rf src
+
+# Now copy real source and build (dependencies already compiled)
+COPY src/ src/
+RUN touch src/main.rs && cargo build --release
 
 # Stage 2: Runtime
 FROM debian:bookworm-slim