Adjust Dockerfile to improve caching

Dockerfile

@@ -1,14 +1,19 @@
 FROM node:8
 
+ENV NODE_ENV=docker
+
 # Create app directory
 WORKDIR /usr/src/app
 
-# Bundle app source
+# Copy package.json into the image, then run yarn install
+# This improves caching so we don't have to download the dependencies every time the code changes
+COPY package.json ./
+# --ignore-scripts tells yarn not to run postbuild.  We run it explicitly later
+RUN yarn install --ignore-scripts
+
+# Bundle app source and build application
 COPY . .
-
+RUN yarn build
-ENV NODE_ENV=docker
-
-RUN yarn
 
 EXPOSE 8000
 CMD [ "yarn", "start" ]