This method works like magic:
https://blog.playmoweb.com/speed-up-your-builds-with-docker-cache-bfed14c051bf
Docker has a special way of caching things for you, and apparently it’s best to use the inborn caching ability.
Cannot say I completely understand how it works, but it does work.
If you follow this pattern, it will work for you:
FROM mhart/alpine-node:5.6.0
WORKDIR /src
# Expose the port 3000
EXPOSE 3000
# Set the default command to run when a container starts
CMD ["node", "server.js"]
# Install app dependencies
COPY package.json /src
RUN npm install
# Copy your code in the docker image
COPY . /src