Docker Interview Guide – Questions & Answers

Docker is an open-source platform that uses containerization technology to package, distribute, and run applications consistently across different computing environments, solving the 'it works on my machine' problem.

Virtualization runs multiple operating systems on a single hardware infrastructure, while containerization shares the host OS kernel, making containers lighter, faster, and more resource-efficient than virtual machines.

Docker architecture consists of the Docker Client, Docker Daemon, Docker Registry, Docker Objects (Images, Containers), and Docker Host, which work together to build, manage, and run containers.

Docker provides consistency, portability, efficiency, rapid deployment, scalability, isolation, version control, and easy environment replication for applications.

A Dockerfile is a text file containing instructions to build a Docker image, specifying the base image, dependencies, environment configurations, and commands needed to create a reproducible container environment.

Docker enables microservices by allowing each service to be packaged in a separate container, providing independent deployment, scalability, and technology stack flexibility.

Container orchestration is the automated management of containerized applications, handling deployment, scaling, networking, load balancing, and ensuring high availability across multiple hosts.

CMD provides default command and parameters for an executing container, which can be overridden, while ENTRYPOINT defines the primary command that always runs when the container starts, with optional CMD as default parameters.

A Docker image is a read-only template containing instructions for creating a container, while a container is a runnable instance of an image with a writable layer that can be started, stopped, and deleted.

The Docker daemon (dockerd) is a background service that manages Docker objects like images, containers, networks, and volumes, listening for Docker API requests and managing Docker resources.

Docker can be installed via official Docker Desktop for Windows and Mac, package managers like apt or yum for Linux, using Docker's official repositories, or through script-based installations.

Run 'docker --version' to check Docker version, 'docker info' for system-wide information, and 'docker run hello-world' to confirm Docker can download and run a test container.

Requirements include a 64-bit operating system, minimum 4GB RAM, hardware virtualization support, kernel version 3.10+ for Linux, and specific OS-dependent prerequisites.

Docker CE is free, open-source, suitable for developers and small teams, while Docker EE offers advanced security, management tools, and support for enterprise environments.

Use 'systemctl enable docker' on systemd-based Linux distributions, or configure Docker Desktop settings on Windows and Mac to ensure automatic startup.

Docker creates three default networks: bridge (default for containers), host (direct host network access), and none (no network interface), which can be customized as needed.

Add your user to the docker group using 'sudo usermod -aG docker $USER' to run Docker commands without sudo, then log out and log back in or restart the system.

Keep Docker and host system updated, use official images, implement least privilege principles, enable Docker content trust, use Docker bench for security, and configure network restrictions.

Uninstallation method varies by OS, but typically involves removing Docker packages, deleting Docker directories, removing user configurations, and cleaning up associated files and groups.

Docker Toolbox, Docker Compose, Portainer, Docker Desktop, and cloud-managed Kubernetes services like EKS, AKS, and GKE can help manage Docker environments.

Create a Dockerfile with build instructions, then use 'docker build -t image_name:tag .' to build an image, or pull an existing image from Docker Hub using 'docker pull'.

Docker reads Dockerfile instructions sequentially, creating a new layer for each instruction, caching intermediate layers to optimize subsequent builds and reduce build time.

Key instructions include FROM (base image), RUN (execute commands), COPY/ADD (copy files), WORKDIR (set working directory), ENV (set environment variables), EXPOSE (network ports), CMD/ENTRYPOINT (default commands).

Use 'docker images' to list images, 'docker rmi' to remove images, 'docker tag' to rename images, and 'docker save/load' to export and import images between systems.

'docker run' creates and starts a new container, 'docker start' restarts a stopped container, and 'docker exec' runs a command in an already running container.

Use commands like 'docker create', 'docker start', 'docker stop', 'docker restart', 'docker pause', 'docker unpause', and 'docker rm' to manage container states and lifecycles.

Docker images are composed of read-only layers, with each instruction creating a new layer. Layers are cached and shared between images, reducing storage and download times.

Multi-stage builds allow using multiple FROM statements in a Dockerfile, enabling separation of build-time and runtime environments, resulting in smaller final images.

Use 'docker inspect' to view detailed information, 'docker logs' to see container logs, 'docker port' to check port mappings, and 'docker top' to view running processes.

Docker Hub is a cloud-based registry for sharing and managing Docker images, allowing users to push, pull, and collaborate on container images publicly or privately.

Docker provides bridge (default), host, none, and user-defined networks, each with specific use cases for container communication and isolation.

Use 'docker network create' to create networks, 'docker network ls' to list networks, and 'docker network connect/disconnect' to manage container network associations.

Port mapping allows exposing container ports to the host system using '-p host_port:container_port', enabling external access to containerized services.

Bridge networks connect containers on a single host, while overlay networks enable communication between containers across multiple Docker daemon hosts.

Containers in the same network can communicate using container names as hostnames, resolving each other through Docker's embedded DNS server.

Use user-defined networks, limit exposed ports, implement network segmentation, use Docker's built-in network encryption, and regularly audit network configurations.

Use commands like 'docker network inspect', 'docker logs', 'docker ps', and network diagnostic tools like ping and netstat to identify and resolve networking problems.

Docker provides an internal DNS service that allows containers to resolve each other by name within user-defined networks, simplifying inter-container communication.

Macvlan allows containers to have their own MAC address, appearing as physical devices on the network, useful for scenarios requiring direct network integration.

Create separate user-defined networks, control network attachments, use network policies, and leverage Docker's built-in network segmentation capabilities.

Docker volumes are persistent data storage mechanisms that exist independently of container lifecycles, enabling data preservation and sharing between containers.

Bind mounts map host filesystem directories, volumes are managed by Docker, and tmpfs mounts store data in host system memory, each with unique use cases and performance characteristics.

Use 'docker volume create', 'docker volume ls', 'docker volume inspect', and 'docker volume rm' to manage volumes, specifying mount points in Dockerfiles or runtime configurations.

Use named volumes, avoid storing data in container layers, implement backup strategies, leverage volume drivers, and separate persistent data from container images.

Use docker cp, volume backup containers, or third-party tools to create volume snapshots, and restore by creating new volumes and copying data.

Volume plugins extend Docker's storage capabilities, enabling integration with external storage systems like cloud storage, network-attached storage, and distributed file systems.

Docker supports multiple storage drivers like overlay2, aufs, btrfs, and devicemapper, each with different performance and compatibility characteristics.

Use multi-stage builds, .dockerignore files, compress files before copying, leverage volume mounts, and implement efficient caching strategies.

Consider data durability, performance, scalability, backup strategies, network storage options, and compatibility with container orchestration platforms.

Use Docker CLI commands, monitoring tools like cAdvisor, Prometheus, and Grafana, and implement logging and metrics collection for storage-related insights.

Docker Compose is a tool for defining and running multi-container Docker applications using a YAML file, enabling easy configuration, scaling, and management of complex application stacks.

A Docker Compose file defines services, networks, volumes, and other configurations, specifying container images, environment variables, port mappings, dependencies, and scaling options.

Use the 'depends_on' keyword to specify service start order, and implement health checks to ensure dependencies are fully operational before starting dependent services.

Key commands include 'docker-compose up', 'docker-compose down', 'docker-compose ps', 'docker-compose logs', 'docker-compose scale', and 'docker-compose build'.

Use environment files (.env), inline environment variables, environment sections in compose files, and external secret management tools for secure configuration.

Docker Compose automatically creates a default network, allows custom network definitions, and enables service-to-service communication using service names as hostnames.

Separate concerns, use environment-specific compose files, leverage service health checks, implement proper volume management, and use version control for compose configurations.

Use 'docker-compose up --scale' command, implement load balancing, leverage Docker Swarm or Kubernetes for advanced orchestration, and design stateless services.

Compose profiles allow selectively starting services, enabling different configurations for development, testing, and production environments within the same compose file.

Use 'docker-compose logs', 'docker-compose ps', interactive debugging, volume mounts for troubleshooting, and leverage built-in health check mechanisms.

Key principles include minimizing image attack surface, implementing least privilege, using trusted images, keeping systems updated, network segmentation, and continuous monitoring.

Use official images, scan images for vulnerabilities, implement multi-stage builds, minimize image layers, remove unnecessary packages, and use image signing and verification.

Container isolation prevents containers from interfering with each other and the host system, using namespaces, control groups, and security options to limit container capabilities.

Tools like Clair, Trivy, Anchore, and Docker Hub's built-in scanning analyze images for known vulnerabilities, license issues, and configuration risks.

Use Docker's role-based access control (RBAC), implement Linux capabilities, configure SELinux/AppArmor profiles, and manage user namespaces.

Limit container resources, disable unnecessary capabilities, use read-only file systems, implement seccomp profiles, and use security options like no-new-privileges.

Docker secrets provide a secure way to manage sensitive information like passwords and tokens, encrypting and distributing secrets across the Docker swarm.

Use user-defined networks, implement network policies, enable encrypted overlay networks, limit exposed ports, and use network segmentation strategies.

Monitor container processes, implement resource limits, use security-enhanced Linux (SELinux), configure AppArmor profiles, and regularly update container runtimes.

Use tools like Sysdig Falco, Docker Enterprise's security scanning, implement logging, set up alerts, conduct regular vulnerability assessments, and automate security checks.

Docker Swarm is Docker's native clustering and orchestration solution, providing built-in load balancing, service discovery, and scaling with simpler setup compared to Kubernetes.

Docker Swarm consists of manager nodes (cluster management), worker nodes (run services), services, tasks, and a distributed key-value store for cluster state management.

Use 'docker swarm init' to create a cluster, 'docker swarm join' to add nodes, 'docker node ls' to list nodes, and manage node roles and availability.

Docker services are scalable definitions of tasks run across a Swarm cluster, allowing easy replication, rolling updates, and load balancing across multiple container instances.

Deploy services using 'docker service create', scale services with 'docker service scale', implement rolling updates, and define replication strategies.

Docker Swarm provides built-in DNS-based service discovery, allowing services to communicate using service names, with automatic load balancing and high availability.

Docker Swarm supports overlay networks for inter-service communication, enabling encrypted communication and network segmentation across multiple nodes.

Use multiple manager nodes, implement quorum-based consensus, configure service constraints, and design stateless, easily replaceable services.

Use infrastructure as code, implement proper monitoring, design stateless services, leverage secrets management, and regularly update and patch cluster nodes.

Use tools like Kompose for YAML conversion, redesign services for Kubernetes architecture, migrate volumes and configurations, and gradually transition workloads.

Use multi-stage builds, minimize base image size, leverage .dockerignore, combine RUN commands, remove unnecessary files, and use Alpine Linux-based images.

Implement resource limits, use host network mode, optimize application code, leverage build cache, minimize layer count, and choose appropriate storage drivers.

Docker build cache stores intermediate layers, reducing build time by reusing unchanged layers, with strategies to invalidate and optimize caching during image builds.

Use cAdvisor, Prometheus, Grafana, Docker stats command, system monitoring tools, and specialized container performance analysis platforms.

Check network configurations, inspect container logs, verify port mappings, use network diagnostic tools, and validate container network connectivity.

Minimize image size, optimize Dockerfile, use lightweight base images, implement multi-stage builds, and leverage build cache effectively.

Monitor CPU, memory, and I/O usage, set appropriate resource limits, use Docker stats, implement health checks, and analyze container performance metrics.

Choose appropriate storage drivers, use volume plugins, implement caching strategies, leverage host filesystem performance, and minimize I/O overhead.

Image size, layer count, storage driver performance, network overhead, resource allocation, application design, and container runtime configuration.

Use monitoring tools, implement logging, set up alerting, conduct regular performance audits, and integrate performance testing into CI/CD pipelines.