Avoid doing an `npm install` after an `ADD .` command

Bad-Dockerfile

FROM ubuntu

RUN echo "deb http://archive.ubuntu.com/ubuntu precise main universe" > /etc/apt/sources.list
RUN apt-get update
RUN apt-get -y install python-software-properties git build-essential
RUN add-apt-repository -y ppa:chris-lea/node.js
RUN apt-get update
RUN apt-get -y install nodejs

WORKDIR /opt/app

ADD . /opt/app
RUN npm install
EXPOSE 3001

CMD ["node", "server.js"]