create-app: sync backend Dockerfile

packages/create-app/templates/default-app/packages/backend/Dockerfile

@@ -2,8 +2,15 @@ FROM node:12
 
 WORKDIR /usr/src/app
 
-COPY . .
+# Copy repo skeleton first, to avoid unnecessary docker cache invalidation.
+# The skeleton contains the package.json of each package in the monorepo,
+# and along with yarn.lock and the root package.json, that's enough to run yarn install.
+ADD yarn.lock package.json skeleton.tar ./
 
 RUN yarn install --frozen-lockfile --production
 
+# This will copy the contents of the dist-workspace when running the build-image command.
+# Do not use this Dockerfile outside of that command, as it will copy in the source code instead.
+COPY . .
+
 CMD ["node", "packages/backend"]