Docker containerization in NestJS - Mini Project: Build & Apply

Docker containerization with NestJS

📖 Scenario: You are building a simple NestJS application that you want to run inside a Docker container. Docker helps package your app with everything it needs so it runs the same on any computer.Imagine you want to share your NestJS app with a friend who does not have Node.js installed. Docker makes this easy by creating a container that includes Node.js and your app together.

🎯 Goal: Create a Dockerfile to containerize a basic NestJS application. You will start by setting up the base image, then add configuration for the app, copy the app files, install dependencies, build the app, and finally specify how to run it inside the container.

📋 What You'll Learn

Use the official Node.js 18 image as the base image

Set the working directory inside the container to /usr/src/app

Copy package.json and package-lock.json to the container

Run npm install to install dependencies

Copy the rest of the application files

Run npm run build to compile the NestJS app

Expose port 3000

Set the command to start the app with npm run start:prod

💡 Why This Matters

🌍 Real World

Docker containerization helps developers package their NestJS apps so they run consistently on any machine or cloud service without setup issues.

💼 Career

Knowing how to containerize backend applications with Docker is a valuable skill for backend developers, DevOps engineers, and cloud engineers.

1

Set the base image and working directory

Create a Dockerfile and write these two lines: use node:18 as the base image with FROM node:18, and set the working directory inside the container to /usr/src/app using WORKDIR /usr/src/app.

NestJS

FROM node:18
# Set working directory
WORKDIR /usr/src/app
# Your code here

Hint

Use FROM to pick the base image and WORKDIR to set the folder inside the container.

2

Copy package files and install dependencies

Add these lines to your Dockerfile: copy package.json and package-lock.json to the container with COPY package*.json ./, then run npm install to install dependencies using RUN npm install.

NestJS

FROM node:18
WORKDIR /usr/src/app
# Copy package files
COPY package*.json ./
# Install dependencies
RUN npm install
# Your code here

Hint

Use COPY to copy files and RUN to execute commands inside the container.

3

Copy app files and build the NestJS app

Add these lines to your Dockerfile: copy all other app files with COPY . ., then build the app using RUN npm run build.

NestJS

FROM node:18
WORKDIR /usr/src/app
COPY package*.json ./
RUN npm install
# Copy app files
COPY . .
# Build the app
RUN npm run build
# Your code here

Hint

Copy all files with COPY . . and build the app with npm run build.

4

Expose port and set the start command

Add these final lines to your Dockerfile: expose port 3000 with EXPOSE 3000, and set the command to start the app in production mode using CMD ["npm", "run", "start:prod"].

NestJS

FROM node:18
WORKDIR /usr/src/app
COPY package*.json ./
RUN npm install
COPY . .
RUN npm run build
# Expose port
EXPOSE 3000
# Start the app
CMD ["npm", "run", "start:prod"]
# Your code here

Hint

Use EXPOSE to open the port and CMD to set the start command.

Practice

1. What is the main purpose of using Docker with a NestJS application?

easy

A. To replace the need for a database in the app

B. To package the app and its environment so it runs the same everywhere

C. To write the NestJS application code faster

D. To automatically generate API documentation

Solution

1. Step 1: Understand Docker's role

   Docker packages an app with all its dependencies and environment settings.

2. Step 2: Apply to NestJS context

   This means the NestJS app runs consistently on any machine with Docker.

3. Final Answer:

   To package the app and its environment so it runs the same everywhere -> Option B

4. Quick Check:

   Docker ensures consistent app environment [OK]

Hint: Docker ensures app runs same everywhere by packaging environment [OK]

Common Mistakes:

• Thinking Docker speeds up coding
• Confusing Docker with database tools
• Assuming Docker auto-generates docs

2. Which of the following is the correct first line in a Dockerfile for a NestJS app using Node 18?

easy

A. USE node@18

B. RUN node install 18

C. FROM node:18-alpine

D. INSTALL node version 18

Solution

1. Step 1: Identify Dockerfile base image syntax

   The Dockerfile starts with FROM followed by the image name and tag.

2. Step 2: Match Node 18 Alpine image

   Correct syntax is 'FROM node:18-alpine' to use Node 18 on Alpine Linux.

3. Final Answer:

   FROM node:18-alpine -> Option C

4. Quick Check:

   Dockerfile base image starts with FROM [OK]

Hint: Dockerfile base image line always starts with FROM [OK]

Common Mistakes:

• Using RUN or INSTALL instead of FROM
• Incorrect syntax like USE node@18
• Confusing Dockerfile commands with shell commands

3. Given this Dockerfile snippet for a NestJS app:

FROM node:18-alpine
WORKDIR /app
COPY package*.json ./
RUN npm install
COPY . .
CMD ["npm", "run", "start"]

What happens when you build and run this container?

medium

A. The container fails because 'COPY' is used incorrectly

B. The container runs but does not start the app

C. The app runs but without installing dependencies

D. The app installs dependencies and starts with 'npm run start'

Solution

1. Step 1: Analyze Dockerfile commands

   WORKDIR sets folder, COPY copies package files, RUN installs dependencies, COPY copies rest, CMD starts app.

2. Step 2: Understand container behavior

   Dependencies are installed before copying source, then app starts with 'npm run start'.

3. Final Answer:

   The app installs dependencies and starts with 'npm run start' -> Option D

4. Quick Check:

   RUN npm install before CMD start [OK]

Hint: RUN npm install before CMD starts app means dependencies installed [OK]

Common Mistakes:

• Thinking COPY package*.json copies whole app
• Assuming dependencies install after app start
• Confusing CMD with RUN commands

4. You wrote this Dockerfile for your NestJS app:

FROM node:18-alpine
WORKDIR /app
COPY . .
RUN npm install
CMD ["npm", "start"]

But when running the container, the app crashes with 'Cannot find module'. What is the likely problem?

medium

A. You copied all files before running npm install, causing missing dependencies

B. The CMD syntax is incorrect and should be a string, not array

C. You forgot to expose the app port in Dockerfile

D. Node 18-alpine image does not support NestJS apps

Solution

1. Step 1: Check COPY and RUN order

   COPY . . copies all files including node_modules if present, then RUN npm install may overwrite or fail.

2. Step 2: Understand missing module cause

   If dependencies are not installed properly before copying, app modules may be missing causing crash.

3. Final Answer:

   You copied all files before running npm install, causing missing dependencies -> Option A

4. Quick Check:

   Order of COPY and RUN npm install matters [OK]

Hint: Run npm install before copying full source to avoid missing modules [OK]

Common Mistakes:

• Blaming CMD syntax for module errors
• Ignoring port exposure as cause
• Assuming Node 18-alpine is incompatible

5. You want to optimize your NestJS Dockerfile to reduce build time by caching dependencies. Which Dockerfile order is best?

hard

A. COPY package*.json ./ then RUN npm install then COPY . . then CMD start

B. COPY . . then RUN npm install then CMD start

C. RUN npm install then COPY package*.json ./ then COPY . . then CMD start

D. CMD start then COPY package*.json ./ then RUN npm install then COPY . .

Solution

1. Step 1: Understand Docker layer caching

   Docker caches layers; copying package files and installing dependencies first caches npm install step.

2. Step 2: Apply best practice order

   Copy package*.json, run npm install, then copy rest of source to avoid reinstalling dependencies on code changes.

3. Final Answer:

   COPY package*.json ./ then RUN npm install then COPY . . then CMD start -> Option A

4. Quick Check:

   Cache dependencies by copying package files first [OK]

Hint: Copy package files first to cache npm install step [OK]

Common Mistakes:

• Copying all files before npm install wastes cache
• Running npm install before copying package files
• Placing CMD before install steps