What is Docker and what problem does it solve?

Docker is a containerization platform that packages applications with dependencies into isolated containers. It solves the 'works on my machine' problem by ensuring consistent environments across development, testing, and production.

What is the difference between a Docker image and container?

An image is a read-only template containing application code, runtime, libraries, and dependencies. A container is a running instance of an image - you can create multiple containers from the same image, like objects from a class.

What is a Dockerfile and what is it used for?

A Dockerfile is a text file with instructions for building Docker images. Each instruction creates a layer in the image. Common instructions include FROM, RUN, COPY, WORKDIR, EXPOSE, and CMD.

What is the difference between CMD and ENTRYPOINT in a Dockerfile?

CMD provides default arguments that can be overridden at runtime. ENTRYPOINT defines the main executable that always runs. Use ENTRYPOINT for the command and CMD for default arguments that users can override.

What is Docker Compose and when should you use it?

Docker Compose is a tool for defining and running multi-container applications using YAML files. Use it when your application needs multiple services (app, database, cache) that need to communicate and be managed together.

What is a multi-stage build in Docker?

Multi-stage builds use multiple FROM instructions to create separate stages. Build tools and dependencies stay in earlier stages while only runtime artifacts are copied to the final minimal image, dramatically reducing image size.

How do Docker image layers work?

Each Dockerfile instruction creates a read-only layer. Layers are cached and reused across images that share the same base. When running containers, Docker adds a writable layer on top. This enables efficient storage and faster builds.

Docker Hub is the default public registry for Docker images. It hosts official images (nginx, postgres, node) and community images. You can also push your own images to share with others or use private repositories.

Why should you avoid using the 'latest' tag in production?

The 'latest' tag is mutable and doesn't guarantee the newest version. Using it makes deployments non-reproducible and breaks rollback strategies. Always use specific version tags for production to ensure consistency.

What is the difference between COPY and ADD in a Dockerfile?

COPY simply copies files from host to image. ADD has extra features: auto-extracting tar archives and downloading from URLs. Best practice is to use COPY for transparency and only ADD when you need tar extraction.

How do Docker networks work?

Docker creates virtual networks for container communication. Bridge is the default for single-host networking. Containers on the same network can communicate by name. Host network shares the host's network stack directly.

Why should containers run as non-root users?

Running as root inside containers is a security risk - if the container is compromised, attackers have root access. Create a non-root user with USER instruction and run your application with minimal required permissions.

Docker Interview Questions

Q: What is the difference between Docker containers and virtual machines?

Containers share the host OS kernel and virtualize at the application layer, making them lightweight (start in seconds). VMs include a full guest OS and virtualize at the hardware layer, making them heavier but providing stronger isolation.

Q: What are Docker volumes and why are they important?

Volumes are the preferred way to persist data generated by containers. They're managed by Docker, exist outside the container lifecycle, and allow data sharing between containers. Without volumes, data is lost when containers are removed.