Re-order dockerfile in example backend slightly

Signed-off-by: Tim Jacomb <21194782+timja@users.noreply.github.com>

packages/backend/Dockerfile

@@ -19,10 +19,8 @@ RUN apt-get update && \
     yarn config set python /usr/bin/python3
 
 # From here on we use the least-privileged `node` user to run the backend.
-WORKDIR /app
-RUN chown node:node /app
 USER node
-
+WORKDIR /app
 
 # This switches many Node.js dependencies to production mode.
 ENV NODE_ENV production