Staged Docker builds for dev and prod

backend/Dockerfile

@@ -1,5 +1,5 @@
 # pull official node image
-FROM node
+FROM node AS dev
 
 # define /app as working directory
 WORKDIR /app
@@ -13,4 +13,10 @@ RUN npm install
 COPY . /app
 
 # launch node server
+ENTRYPOINT npm run dev
+
+# production
+FROM node AS prod
+WORKDIR /app
+COPY --from=dev /app ./
 ENTRYPOINT npm run start