Interview Preparation

Docker Interview Questions

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Q1
What is Docker and what are containers?

Docker is an open platform for developing, shipping, and running applications in isolated environments called containers.

Containers are lightweight, standalone, executable packages that include everything needed to run an application: code, runtime, system tools, libraries, and settings.

Containers are isolated from one another and the host system, yet share the operating system kernel, making them more efficient than virtual machines.

Docker simplifies development, testing, and deployment by ensuring consistency across environments.

Q2
What is the difference between Docker and virtual machines?

Virtual machines (VMs) emulate hardware and run a full OS, consuming more resources and taking longer to boot.

Docker containers share the host OS kernel and isolate processes, making them lightweight and fast to start.

Containers are portable and reproducible, making them ideal for microservices and CI/CD workflows.

VMs are better for running full OS-level applications, while Docker is ideal for deploying application-level services.

Q3
What is a Dockerfile and how is it used?

A Dockerfile is a text document that contains all the commands a user could call on the command line to assemble an image.

It starts with a base image and adds layers for installing dependencies, copying files, exposing ports, and defining entry points.

Images are built using docker build and containers are created from images using docker run.

Dockerfiles enable automated builds and consistent containerized environments across development, testing, and production.

Q4
What is the difference between CMD and ENTRYPOINT in a Dockerfile?
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CMD provides defaults that can be overridden at runtime, while ENTRYPOINT specifies the executable that will run when the container starts.

ENTRYPOINT defines the container's main command and keeps it immutable, while CMD defines default arguments that can be replaced on the CLI.

Combining ENTRYPOINT and CMD allows defining a fixed command with customizable arguments.

ENTRYPOINT is ideal for executable containers, while CMD is useful for providing default parameters.

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