Introduction

Docker has revolutionized the way we build, package, and deploy applications. By containerizing applications, we ensure consistent behavior across different environments, be it a developer's local machine 🖥️, a staging environment, or production 🚀.

Problem Statement 🤔

Imagine working on an application on your local machine, and everything works perfectly. However, when you deploy it to production, things break 😖. This inconsistency between environments is often summarized by the phrase: "But it works on my machine!"

Docker addresses this problem by allowing you to package your application along with all its dependencies into a consistent environment known as a container. This container can be run uniformly across different platforms 🌍.

Objectives

By the end of this lesson, you should be able to:

1. Understand the basics of Docker.

2. Containerize an Express app 📦.

3. Run the containerized app on your local machine.

What is Docker? 🐳

Docker is a platform that enables developers to create, deploy, and run applications in containers. A container is a standardized unit of software that contains everything the software needs to run: code, runtime, system tools, system libraries, and settings.

Steps to Containerise the Express App 🛠️

1. Install Docker 🔧

Before you can use Docker, you need to install it on your machine. Visit Docker's official website and follow the instructions for your operating system.

2. Create a Dockerfile 📄

A Dockerfile is a script with instructions on how to build a Docker image.

First of all, Create a folder called docker inside the root of the project. Then inside it, create a folder called development. In this folder we will create our Dockerfile. For our Express app, the Dockerfile might look something like:

# Use the official Node.js image as our base
FROM node:18

# Set the working directory inside the container
WORKDIR /usr/src/app

# Copy package.json and package-lock.json first to leverage Docker cache
COPY package*.json ./

# Install app dependencies
RUN npm install

# Copy the rest of our app's source code into the container
COPY . .

# Expose the port the app will run on
EXPOSE 5501

# The command to run our app
CMD ["npm", "run", "dev"]

3. .dockerignore File 🚫

To ensure that local node modules and debug logs aren't copied into our Docker image, we'll use a .dockerignore file:

node_modules
npm-debug.log
.env

4. Building the Docker Image 🏗️

Navigate to the directory containing your Dockerfile and run:

docker build -t auth-service:dev -f docker/development/Dockerfile .

This command will produce a Docker image named auth-service with the tag dev.

5. Running the Express App in a Docker Container 🚀

Once the image is built, you can run it:

docker run --rm -it -v $(pwd):/usr/src/app -v /usr/src/app/node_modules --env-file $(pwd)/.env -p 5501:5501 -e NODE_ENV=development auth-prep:dev

ℹ️ For Powershell users: use ${PWD} instead of $(pwd)

ℹ️ For Command prompt (cmd) users: use %cd% instead of $(pwd)

Your Express app is now accessible at http://localhost:5501.

6. Stoping the Docker Container ✋🏻

You can stop the running container by using this command:

// If container is running in interactive mode.
ctr + c

// If container is running in detached mode.
// List all running container
docker ps

// Stop the container using container id
docker stop <container id>

Conclusion 🌟

Docker provides a consistent environment for applications, reducing the "it works on my machine" problem. With your Express app now containerized, you can be confident that it will run the same way everywhere Docker is installed. As you proceed in your MERN stack journey, consider containerizing other parts of the stack for a seamless development and deployment experience.