How to cache the RUN npm install instruction when docker build a Dockerfile

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Ok so I found this great article about efficiency when writing a docker file.

This is an example of a bad docker file adding the application code before running the RUN npm install instruction:

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

COPY . /opt/app
RUN npm install
EXPOSE 3001

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

By dividing the copy of the application into 2 COPY instructions (one for the package.json file and the other for the rest of the files) and running the npm install instruction before adding the actual code, any code change wont trigger the RUN npm install instruction, only changes of the package.json will trigger it. Better practice docker file:

FROM ubuntu
MAINTAINER David Weinstein <[email protected]>

# install our dependencies and nodejs
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

# use changes to package.json to force Docker not to use the cache
# when we change our application's nodejs dependencies:
COPY package.json /tmp/package.json
RUN cd /tmp && npm install
RUN mkdir -p /opt/app && cp -a /tmp/node_modules /opt/app/

# From here we load our application's code in, therefore the previous docker
# "layer" thats been cached will be used if possible
WORKDIR /opt/app
COPY . /opt/app

EXPOSE 3000

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

This is where the package.json file added, install its dependencies and copy them into the container WORKDIR, where the app lives:

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

To avoid the npm install phase on every docker build just copy those lines and change the ^/opt/app^ to the location your app lives inside the container.

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