Question 1

What is the difference between a container and a virtual machine?

Accepted Answer

A virtual machine runs a full guest operating system on a hypervisor, so it is heavy and slow to start. A container shares the host's Linux kernel and only packages the app and its libraries, using namespaces for isolation and cgroups for resource limits. That makes containers small, fast to start and easy to run many per server. VMs give stronger isolation, which is why many clouds run containers inside VMs.

Question 2

What is the difference between a Docker image and a container?

Accepted Answer

An image is a read-only template made of layers: the app, its dependencies and the start command. A container is a running instance of an image with a thin writable layer on top. One image can start many containers, just like one class can create many objects.

Question 3

Walk me through a simple Dockerfile for a Node.js app.

Accepted Answer

I start FROM a small official base like node:20-alpine, set WORKDIR /app, COPY package*.json first and run npm ci, so the dependency layer is cached. Then I COPY the rest of the code, EXPOSE the port, switch to a non-root USER and set CMD to start the app. I add a .dockerignore so node_modules and secrets are not copied in.

Question 4

How do containers keep data when they are deleted?

Accepted Answer

A container's own filesystem is lost when it is removed, so data that must last goes into a volume or a bind mount. A named volume is managed by Docker and is the best choice for databases. A bind mount maps a host folder into the container and is handy in development. In Kubernetes the same idea is a PersistentVolume.

Question 5

What is the difference between CMD and ENTRYPOINT?

Accepted Answer

ENTRYPOINT sets the main program the container always runs. CMD gives default arguments, or a default command, that can easily be overridden at docker run. A common pattern is ENTRYPOINT ["python", "app.py"] with CMD ["--port", "8080"], so users can change the arguments but not the program.

Docker interview questions