feat(docker): optimize build caching and fix binary path

- Copy Cargo files first to leverage Docker layer caching
- Build dependencies separately before copying source code
- Use cache mounts for cargo registry to speed up builds
- Fix binary path in final stage from cargo bin to target release

Dockerfile

@@ -4,12 +4,24 @@ FROM rust:1.91 AS builder
 WORKDIR /usr/src/pop_imap_importer
 
 # Copy manifests first to leverage Docker layer caching
-COPY src/ Cargo.* ./
+COPY Cargo.* ./
+
+# Create a dummy src/main.rs file to build dependencies
+RUN mkdir -p src && \
+    echo "fn main() {}" > src/main.rs
+
+# Build dependencies only - this will be cached as long as Cargo.toml/Cargo.lock don't change
+RUN --mount=type=cache,target=/usr/local/cargo/registry \
+    cargo build --release
+
+# Copy the actual source code
+COPY src src
+RUN touch src/main.rs
 
 # Build the actual application
-RUN cargo build --release
-
-RUN cargo install --path .
+RUN --mount=type=cache,target=/usr/local/cargo/registry \
+    ls -lh target/release && \
+    cargo build --release
 
 # Runtime stage
 FROM debian:bookworm-slim
@@ -20,7 +32,7 @@ RUN apt-get update && \
     rm -rf /var/lib/apt/lists/*
 
 # RUN apt-get update && apt-get install -y extra-runtime-dependencies && rm -rf /var/lib/apt/lists/*
-COPY --from=builder /usr/local/cargo/bin/pop_imap_importer /usr/local/bin/pop_imap_importer
+COPY --from=builder /usr/src/pop_imap_importer/target/release/rs_pop_imap_importer /usr/local/bin/pop_imap_importer
 
 CMD ["pop_imap_importer"]