testProject

Dockerfile

FROM ubuntu
MAINTAINER David Weinstein <david@bitjudo.com>

# 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:
ADD 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
ADD . /opt/app

EXPOSE 3000

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

package.json

{
  "name": "testProject",
  "description": "this project is awesome",
  "version": "0.0.1",
  "private": true,
  "scripts": {
    "start": "node server.js"
  },
  "dependencies": {
    "async": "*"
  }
}

server.js

// this is our simulated application code
//
var async=require('async');

// uncomment the following line after building the project
// docker will use the cache for the modules...
// console.log('hello');

How about making all the RUN commands in a single statement
Instead of

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

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

@onlinejudge95 You can do that as well but this would result in whole image building process to boil down to a single layer.
This multi layer building concept which is being used in multi statement format has many advantages, as explained in this blog article.
For more info refer this official documentation.