Fix syntheticbucketd dockerfile to support caching deps in layer

Issue: BB-295

Author: francoisferrand

tests/utils/syntheticbucketd/Dockerfile

@@ -1,7 +1,10 @@
 FROM node:16
 
 RUN mkdir /app
-COPY * /app
+COPY package.json /app
+COPY yarn.lock /app
 RUN cd /app && yarn install --network-concurrency 1
 
+COPY * /app
+
 CMD ["node", "/app/syntheticbucketd.js", "200000", "1000"]