Docker containerization in Express - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when you run this Dockerfile?

Consider this Dockerfile for an Express app. What will be the output when you build and run the container?

Express

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

A The container starts and the Express app listens on port 3000 inside the container.

B The container builds but crashes immediately because server.js is missing.

C The container starts but the app listens on port 80 by default.

D The container fails to build because the WORKDIR is missing a slash.

💡 Hint

Check the EXPOSE and CMD instructions and what they do.

The Dockerfile sets the working directory, installs dependencies, copies files, exposes port 3000, and runs the app with node server.js. This means the app listens on port 3000 inside the container.

📝 Syntax

intermediate

Identify the syntax error in this Dockerfile snippet

Which option correctly identifies the syntax error in this Dockerfile snippet?

Express

FROM node:18
WORKDIR /app
COPY package.json ./
RUN npm install
COPY . .
EXPOSE 3000
CMD [node server.js]

A COPY . . is invalid because it needs a destination folder.

B RUN npm install is invalid because it should be RUN npm install -g.

C EXPOSE 3000 is invalid because it must be EXPOSE "3000".

D CMD [node server.js] is invalid because CMD requires an array or a string with exec form.

💡 Hint

Look at the CMD instruction format.

CMD should be either an exec form array like ["node", "server.js"] or a shell form string. Writing CMD [node server.js] without quotes or commas causes a syntax error.

🔧 Debug

advanced

Why does the container fail to connect to the Express app?

You built and ran this Docker container, but your browser cannot connect to the Express app on localhost:3000. What is the most likely cause?

Express

Docker run command: docker run -d -p 3000:3000 my-express-app
Express app listens on port 3000
Dockerfile exposes port 3000

A The container port 3000 is mapped to host port 80, so you must use localhost:80.

B The Dockerfile EXPOSE command is missing, so port 3000 is not open.

C The Express app is listening only on localhost (127.0.0.1) inside the container, so external connections fail.

D The docker run command is missing the -it flag, so the app does not start.

💡 Hint

Check the network binding of the Express app inside the container.

If Express listens only on 127.0.0.1, it is not reachable from outside the container. It must listen on 0.0.0.0 to accept external connections.

❓ state_output

advanced

What is the value of NODE_ENV inside the container?

Given this Dockerfile snippet, what will be the value of NODE_ENV inside the running container?

Express

FROM node:18
ENV NODE_ENV=production
WORKDIR /app
COPY . .
RUN npm install
CMD ["node", "server.js"]

A "production"

B "development"

C undefined

D "NODE_ENV"

💡 Hint

Look at the ENV instruction in Dockerfile.

The ENV instruction sets environment variables inside the container. NODE_ENV is set to production.

🧠 Conceptual

expert

Which option best explains the benefit of multi-stage builds in Docker for Express apps?

Why use multi-stage builds in Dockerfiles for Express applications?

A They allow running multiple Express apps in one container simultaneously.

B They reduce the final image size by separating build dependencies from runtime dependencies.

C They enable hot-reloading of Express code inside the container without rebuilding.

D They automatically update the Node.js version inside the container at runtime.

💡 Hint

Think about image size and build vs runtime needs.

Multi-stage builds let you install build tools and dependencies in one stage, then copy only the needed files to a smaller final image, reducing size and attack surface.