Improve Dockerfile image layers to avoid recompiles

Dockerfile

@@ -1,16 +1,12 @@
 # syntax=docker/dockerfile:1
 
-# Comments are provided throughout this file to help you get started.
-# If you need more help, visit the Dockerfile reference guide at
-# https://docs.docker.com/engine/reference/builder/
-
-################################################################################
-# Create a stage for building the application.
 
+# Stage for building backend
 ARG RUST_VERSION=1.71.0
 ARG APP_NAME=cavalier_contours_server
 FROM rust:${RUST_VERSION}-slim-bullseye AS build_backend
 ARG APP_NAME
+WORKDIR /app
 
 # Build the application.
 # Leverage a cache mount to /usr/local/cargo/registry/
@@ -30,39 +26,36 @@ cargo build --locked --release
 cp ./target/release/$APP_NAME /bin/server
 EOF
 
+# Stage for building frontend (Rust needed for building WASM)
 FROM rust:${RUST_VERSION}-slim-bullseye AS build_frontend
 WORKDIR /app
 
+# Install curl
 RUN apt-get update && \
     apt-get install -yq --no-install-recommends \
     curl
 
+# Install node and wasm-pack
 RUN <<EOF
 curl -fsSL https://deb.nodesource.com/setup_20.x | bash -
 apt-get install -y nodejs
 npm install -g wasm-pack
 EOF
 
+# Get dependencies
+COPY ["frontend/package.json", "frontend/package-lock.json*", "./"]
+RUN npm install
+
+# Build wasm
+COPY ["frontend/cavalier_contours_web_ffi/", "./cavalier_contours_web_ffi"]
+RUN npm run wasm
+
+# Vite build
 COPY ["frontend/", "./"]
+RUN npm run build && cp -r ./dist /bin/index
 
-RUN <<EOF
-npm install
-npm run wasm
-npm run build
-cp -r ./dist /bin/index
-EOF
 
-################################################################################
-# Create a new stage for running the application that contains the minimal
-# runtime dependencies for the application. This often uses a different base
-# image from the build stage where the necessary files are copied from the build
-# stage.
-#
-# The example below uses the debian bullseye image as the foundation for running the app.
-# By specifying the "bullseye-slim" tag, it will also use whatever happens to be the
-# most recent version of that tag when you build your Dockerfile. If
-# reproducability is important, consider using a digest
-# (e.g., debian@sha256:ac707220fbd7b67fc19b112cee8170b41a9e97f703f588b2cdbbcdcecdd8af57).
+# Final stage for minimal runtime
 FROM debian:bullseye-slim AS final
 
 # Create a non-privileged user that the app will run under.
@@ -78,7 +71,7 @@ RUN adduser \
     appuser
 USER appuser
 
-# Copy the executable from the "build" stage.
+# Copy files from build stages
 COPY --from=build_backend /bin/server /bin/
 COPY --from=build_frontend /bin/index /bin/index