Install node modules before copying over your working code so that node_modules are built (and cached) before you change your service code!

Dockerfile-nodejs

# ...
ADD package.json /tmp/package.json
RUN cd /tmp && npm install && \
    mkdir -p /opt/app && cp -a /tmp/node_modules /opt/app/

# ...
WORKDIR /opt/app
ADD . /opt/app

If the package.json file changes then Docker will re-run the npm install sequence... otherwise our modules are cached so we aren't rebuilding them every time we change our apps source code!

👍

For those like myself who were initially bewildered by this, the "Build cache" section of the Dockerfile best practices doc may help fill in the gaps.