fix: Remove Cargo.lock from COPY in Dockerfile (it is gitignored)

backend/docker/Dockerfile.dev

@@ -10,14 +10,14 @@ RUN apt-get update && apt-get install -y \
     libssl-dev \
     && rm -rf /var/lib/apt/lists/*
 
-# Copy Cargo.toml (Cargo.lock is optional for dev)
+# Copy Cargo files first for better caching
 COPY Cargo.toml ./
 
 # Create dummy main.rs for dependency caching
 RUN mkdir src && echo 'fn main() {}' > src/main.rs
 
 # Build dependencies (this layer will be cached)
-RUN cargo build
+RUN cargo build && rm -rf src
 
 # Copy actual source code
 COPY src ./src