Docker deployment in NextJS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What will be the output when running this Dockerfile for a Next.js app?

Consider this Dockerfile for a Next.js app. What will be the final behavior when you run the container and visit the app in a browser?

NextJS

FROM node:18-alpine
WORKDIR /app
COPY package.json package-lock.json ./
RUN npm install
COPY . .
RUN npm run build
EXPOSE 3000
CMD ["npm", "start"]

A The app runs in production mode on port 3000, serving the built Next.js app.

B The app runs in development mode with hot reload on port 3000.

C The container fails to start because the build step is missing.

D The app runs but on port 80 instead of 3000.

💡 Hint

Check the CMD and EXPOSE instructions and what npm start does in Next.js.

This Dockerfile installs dependencies, builds the Next.js app, and then runs it in production mode using npm start. The app listens on port 3000 as exposed.

📝 Syntax

intermediate

Which Dockerfile snippet correctly copies only production dependencies for a Next.js app?

You want to optimize your Docker image by installing only production dependencies. Which snippet correctly does this?

A RUN npm install --prod

B RUN npm install --only=production

C RUN npm ci --only=production

D RUN npm install --production=true

💡 Hint

Check npm install flags for production-only installs.

The npm install --only=production flag installs only production dependencies. Other options are either invalid or deprecated.

🔧 Debug

advanced

Why does this Docker container fail to serve the Next.js app after build?

Given this Dockerfile snippet, the container starts but the app shows a 404 error on all routes. What is the likely cause?

NextJS

FROM node:18-alpine
WORKDIR /app
COPY package.json package-lock.json ./
RUN npm install
COPY . .
RUN npm run build
EXPOSE 3000
CMD ["node", "server.js"]

A Port 3000 is not exposed correctly, so the app is unreachable.

B The build step failed silently, so no pages exist to serve.

C The app uses Next.js default server, but CMD runs a non-existent server.js file causing 404.

D The container lacks environment variables needed to run the app.

💡 Hint

Check the CMD instruction and whether server.js exists by default in Next.js.

Next.js apps by default do not have a server.js file unless custom server code is added. Running node server.js causes the app not to start properly, resulting in 404 errors.

❓ lifecycle

advanced

What is the effect of using multi-stage builds in Docker for Next.js deployment?

You use a multi-stage Dockerfile with a build stage and a final stage copying only the build output and node_modules. What benefit does this provide?

A It automatically updates the app when source code changes.

B It speeds up the build process by running all commands in one stage.

C It allows hot reloading inside the container during development.

D It reduces the final image size by excluding development dependencies and build tools.

💡 Hint

Think about what multi-stage builds do with intermediate files.

Multi-stage builds let you separate build dependencies from the final image, keeping only what is needed to run the app. This reduces image size and attack surface.

🧠 Conceptual

expert

Which environment variable setup is best for configuring Next.js runtime behavior in Docker?

You want to configure Next.js to run in production mode and set a custom API URL inside a Docker container. Which approach is correct?

A Set environment variables in the Dockerfile using ENV, then access them in Next.js via process.env.

B Hardcode variables in next.config.js and rebuild the image for each change.

C Pass variables only at runtime with docker run -e but do not declare them in Dockerfile.

D Use a .env file inside the container without declaring ENV or passing variables.

💡 Hint

Consider how Next.js reads environment variables and Docker best practices.

Setting environment variables in the Dockerfile with ENV makes them available at build and runtime. Next.js accesses them via process.env. This is reliable and clear.

Practice

1. What is the main purpose of using Docker for deploying a Next.js application?

easy

A. To write the Next.js code faster

B. To package the app with all dependencies for consistent running anywhere

C. To replace the need for React in the app

D. To automatically generate CSS styles

Solution

1. Step 1: Understand Docker's role

   Docker packages an app with its environment so it runs the same on any machine.

2. Step 2: Relate to Next.js deployment

   This packaging ensures the Next.js app and its dependencies work consistently everywhere.

3. Final Answer:

   To package the app with all dependencies for consistent running anywhere -> Option B

4. Quick Check:

   Docker ensures consistent app environment = A [OK]

Hint: Docker packages app and dependencies for consistent deployment [OK]

Common Mistakes:

• Thinking Docker speeds up coding
• Confusing Docker with React features
• Assuming Docker styles the app automatically

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

easy

A. FROM node:18-alpine

B. RUN node install 18

C. COPY node:18 /app

D. START node version 18

Solution

1. Step 1: Identify Dockerfile base image syntax

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

2. Step 2: Match Node.js 18 base image

   Node official images use 'node:18-alpine' for a lightweight Node 18 environment.

3. Final Answer:

   FROM node:18-alpine -> Option A

4. Quick Check:

   Dockerfile base image starts with FROM = B [OK]

Hint: Dockerfile starts with FROM and image name [OK]

Common Mistakes:

• Using RUN instead of FROM for base image
• Trying to COPY node version as a command
• Using START which is not a Dockerfile command

3. Given this Dockerfile snippet for Next.js deployment:

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

What happens when you run this Docker image?

medium

A. The app builds and starts serving on default port

B. The app installs dependencies but never builds

C. The app builds but fails to start

D. The Dockerfile causes a syntax error

Solution

1. Step 1: Analyze Dockerfile commands

   It sets working directory, copies package.json, installs dependencies, copies all files, builds app, then starts it.

2. Step 2: Understand CMD effect

   CMD runs 'npm start' which serves the built Next.js app on default port 3000.

3. Final Answer:

   The app builds and starts serving on default port -> Option A

4. Quick Check:

   Build then start with npm = C [OK]

Hint: RUN builds, CMD starts app in Dockerfile [OK]

Common Mistakes:

• Assuming build step is missing
• Thinking CMD does not start the app
• Believing Dockerfile syntax is wrong here

4. You wrote this Dockerfile for your Next.js app but get an error when running the container:

FROM node:18-alpine
WORKDIR /app
COPY . .
RUN npm install
RUN npm run build
CMD npm start

What is the likely cause of the error?

medium

A. npm run build should be replaced with npm build

B. COPY command should come after RUN npm install

C. WORKDIR must be set after RUN commands

D. CMD syntax is incorrect; it should be an array

Solution

1. Step 1: Check CMD syntax

   CMD should be an array like ["npm", "start"] to run correctly in Docker.

2. Step 2: Verify other commands

   COPY before npm install is fine; WORKDIR before RUN is correct; npm run build is valid.

3. Final Answer:

   CMD syntax is incorrect; it should be an array -> Option D

4. Quick Check:

   CMD needs array syntax = A [OK]

Hint: Use array syntax for CMD in Dockerfile [OK]

Common Mistakes:

• Writing CMD as a string instead of array
• Moving COPY after npm install unnecessarily
• Changing npm run build to npm build incorrectly

5. You want to deploy a Next.js app with Docker and ensure it listens on port 8080 instead of the default 3000. Which change should you make in your Dockerfile and app start command?

hard

A. Modify Dockerfile to FROM node:8080 and keep app default

B. Change CMD to ["npm", "start", "--port", "8080"] without app changes

C. Set ENV PORT=8080 and run CMD ["npm", "start"] with app configured to use process.env.PORT

D. Expose port 8080 but keep app listening on 3000

Solution

1. Step 1: Set environment variable for port

   Use ENV PORT=8080 in Dockerfile so app can read the port from environment.

2. Step 2: Ensure app uses process.env.PORT

   Next.js app must listen on process.env.PORT to respect Docker port setting.

3. Final Answer:

   Set ENV PORT=8080 and run CMD ["npm", "start"] with app configured to use process.env.PORT -> Option C

4. Quick Check:

   Use ENV and app reads process.env.PORT = D [OK]

Hint: Set ENV PORT and app reads process.env.PORT [OK]

Common Mistakes:

• Changing base image to node:8080 (invalid)
• Exposing port without changing app port
• Passing port as npm start argument without app support