Dockerizing a Node.js app.


Welcome to Metric Tree Labs blog series and in today’s blog, we will be walking you through the basics of docker and how to deploy a node.js app on docker.


WHAT is Docker.


Docker is a service that uses OS-level virtualization to deliver software in packages called containers. Compared to virtual machines that run host operating systems, docker doesn’t need a hypervisor, all it needs is an OS that supports programs, libraries and system resources to run a specific program. And it can run a lot more applications at the same time than what would be possible with a virtual machine. 



  • link to download and install docker.
  • Docker –version and docker info to get version and detailed information about your docker installation.
  • Run docker run hello-world to test the installation.

Node App

  • We have a simple node.js server (app.js) running on port 3000.
  • The server exposes a route “/” that responds with a message saying whether the server is running inside a container or not by looking up the environment variable IN_CONTAINER.
  • You can start the server by running npm start.
  • Visit localhost:3000 on your browser after starting the server.


  • To deploy our server on docker we start by creating a file named Dockerfile.
  • Contents of Dockerfile
    • FROM: The FROM instruction initializes a new build stage and sets the Base Image for subsequent instructions. 
    • WORKDIR: Sets the current working directory for subsequent commands and instructions
    • COPY: copies new files or directories from <src> and adds them to the filesystem of the container at the path
    • RUN: To run commands
    • ENV: Sets the environment variable
    • CMD: Default command to execute. A Dockerfile can have only one CMD instruction
  • We also create a dockerignore file that ignores specified files and folders when copying from our directory to the container.


Creating a Docker image

  • To build the docker image run docker build –tag=<name>.
  • We can list the images using docker image ls command

Running Containers

  • Use the command docker run -p 3000:3000 <image-name> ( -p flag exposes and connects the port 3000 on the container to the system’s port )
  • Visit localhost:3000 in your browser and It should now display the message server running inside a container.
  • To stop the container press Ctrl+c. Containers can be run in the background using the detached mode by adding the flag -d.
  • To view, a list of running containers run docker container ls
  • Stop a container by running docker container stop <container-id>

Deleting Images

  • Run docker image rm <image-id>
  • Run docker image rm <image-id>