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.