Day 21: Important Docker Interview Questions

Day 21: Important Docker Interview Questions

Sep 21, 2023ยท

8 min read

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Table of contents

Docker is a good topic to ask in a DevOps interview, especially for freshers. Must try these questions to gain a better understanding of Docker.


  1. What is the Difference between an Image, Container and Engine?

    • Docker Image:

      An image is a lightweight, standalone package that includes everything to run a piece of software including codes, libraries etc. It serves as a template for the docker container. Images are read-only and used for creating the docker container.

    • Docker Container:

      A Docker container is a running instance of an image. Containers are where applications run and can be started, stopped or deleted independently. It is created from an image.

    • Docker Engine:

      It is a core component of docker. It includes the Docker daemon and CLI, facilitating image creation, container management and interaction with the host OS.

  2. What is the Difference between the Docker command COPY vs ADD?

    • COPY:

      Copies local files and directories from the host to the container. It's straightforward and doesn't unpack files or fetch resources.

    • ADD:

      It can copy local files, extract compressed archives, and fetch resources from URLs.

  3. What is the Difference between the Docker command CMD vs RUN?

    • RUN:
      The RUN command is used to execute commands during the image-building process. It is typically used to install required packages and libraries.

    • CMD:

      The CMD command specifies the default command to run when a container is started from the image.

      It is used to define the primary application that runs within the container.

  4. How will you reduce the size of the Docker image?

    • Use minimal base images like Alpine Linux.

    • Employ multi-stage builds to eliminate build artefacts.

    • Remove unnecessary files and dependencies.

    • Minimize image layers by combining commands.

  5. Why and when to use Docker?

    Use Docker when you need to streamline your development-to-deployment pipeline, deploy microservices or complex applications with multiple dependencies, scale applications horizontally, and ensure consistent behaviour across different environments. It's particularly valuable in cloud-native and DevOps environments for its ability to accelerate application delivery and management.

  6. Explain the Docker components and how they interact with each other.

    Here are the components of docker and their short explanation:

    • Docker Daemon:

      The Docker Client communicates with the Docker Daemon, a background service responsible for container management. It listens to user requests and carries out tasks.

    • Docker Images:

      Docker Images, which are read-only templates, serve as the basis for containers. Images can be built locally or pulled from Docker Registries.

    • Docker Containers:

      Containers are runtime instances of Docker Images. Multiple containers can run from the same image, and they operate in isolated environments.

    • Docker Registries:

      Docker Registries, such as Docker Hub, store and distribute Docker Images. Users can push images to or pull them from these repositories.

    • Docker Compose:

      Docker Compose is used for orchestrating multi-container applications. Configuration details are specified in YAML files, defining services, networks, and volumes.

      All these components interact with the help of Docker CLI which communicates with the docker daemon running on the host system. The Daemon manages containers based on Docker images, which can be stored locally or fetched from the registry. Docker-compose helps manage complex applications by defining their configuration.

  7. Explain the terminology: Docker Compose, Docker File, Docker Image, Docker Container?

    • Docker Compose:

      A tool for defining and running multi-container applications using YAML configuration files. It orchestrates the services, networks, and volumes required for the application.

    • Dockerfile:

      A script used to build Docker Images. It contains instructions on how to create a Docker Image, specifying the base image, application code, dependencies, and configurations.

    • Docker Image:

      A lightweight, standalone, executable package that includes everything needed to run an application, including code, runtime, libraries, and system tools.

    • Docker Container:

      A runtime instance of a Docker Image. Containers are isolated environments where applications run, and they can be created, started, stopped, moved, and deleted with ease.

  8. In what real scenarios have you used Docker?

    There are many real-life scenarios in which docker can be used. Here are some scenarios:

    • Microservices Deployment:

      Docker is widely used to containerize microservices, making it easier to deploy, scale, and manage individual components of a complex application independently.

    • Continuous Integration/Continuous Deployment (CI/CD):

      Docker containers are instrumental in CI/CD pipelines, where applications are built, tested, and deployed in isolated environments, ensuring consistency across different stages.

    • Development Environments:

      Developers use Docker to create reproducible development environments, eliminating the "it works on my machine" problem and streamlining collaboration.

  9. Docker vs Hypervisor?

    • Docker:

      Docker uses containerization, which packages an application and its dependencies together into a single unit called the container.

      Docker containers are lightweight and have low overhead since they share the host OS resources.

      Use Docker when you want to containerize applications for efficient, lightweight, and portable deployments, especially in microservices architectures.

    • Hypervisor:

      Hypervisors, such as VMware or VirtualBox, use hardware virtualization to create virtual machines (VMs) that run full operating systems.

      VMs created by hypervisors are more resource-intensive compared to containers because each VM includes a full OS.

      Use hypervisors when you need to run multiple complete operating systems with strong isolation.

  10. What are the advantages and disadvantages of using docker?


    • Docker containers encapsulate applications and their dependencies, making them highly portable across different environments and platforms.

    • Containers provide process and resource isolation, ensuring that applications run consistently and do not interfere with each other.

    • Docker containers share the host OS kernel, leading to efficient resource utilization and reduced overhead compared to traditional virtualization.


    • Docker introduces complexity, especially when managing containerized applications in production. This complexity includes orchestrating containers, managing networking, and monitoring.

    • Docker has a learning curve, particularly for individuals new to containerization and container orchestration concepts.

    • While more efficient than traditional virtualization, Docker containers still incur some resource overhead compared to running applications directly on the host OS.

  11. What is a Docker namespace?

    A Docker namespace is a technology that provides isolation and separation of resources and objects within a Docker container. Namespaces are a fundamental feature of the Linux kernel, and Docker leverages them to create isolated environments for containers. Each namespace provides a unique view of a specific resource type, such as processes, networks, or filesystems.

  12. What is a Docker registry?

    A Docker registry is a central repository for storing, managing, and distributing Docker images. It serves as a storage and distribution service for Docker images, allowing users to share and deploy containerized applications easily.

  13. What is an entry point?

    In Docker, an "entry point" refers to the command or executable that is run when a container based on a Docker image is started. It defines the default application or process that should be executed within the container.

  14. How to implement CI/CD in Docker?

    Implementing the CI/CD in docker involves automating the building, testing, and deployment of docker containers to streamline software development and delivery. Here are the steps for implementing the CI/CD in Docker:

    • Version Control (Git)

    • Dockerize application (docker file)

    • CI (Jenkins, Travis CI, CircleCI)

    • CD (deployment staging)

    • Image registry (DockerHub, amazon ECR)

    • Orchestration (Kubernetes)

    • Iac (Terraform, AWS cloudformation)

  15. Will data on the container be lost when the docker container exits?

    Data within a Docker container is ephemeral, which means it can be lost when the container exits. This is because Docker containers are designed to be lightweight, stateless, and isolated environments that start from a clean slate each time they run.

  16. What is a Docker swarm?

    Docker Swarm is a native clustering and orchestration solution for Docker. It turns a group of Docker hosts into a single virtual Docker host, making it easy to manage and scale containerized applications across multiple machines.

    Here are a few characteristics of Docker Swarm:

    • Cluster Management

    • Service Orchestration

    • Load Balancing

    • High availability

    • Integration with docker CLI

  17. What are the common Docker practices to reduce the size of Docker Images?

    Here are the common Docker practices to reduce the size of Docker images:

    • Use a minimal base image like Alpine Linux.

    • Employ multi-stage builds to discard unnecessary build dependencies.

    • Remove temporary files and caches within the Dockerfile.

    • Optimize layers by combining commands.

    • Choose smaller versions of dependencies.

    • Execute cleanup commands to remove package caches.

    • Consider Alpine Linux for minimalism.

    • Use a .dockerignore file to exclude unneeded files.

  18. What are the docker commands for the following:

    view running containers

    command to run the container under a specific name

    command to export a docker

    command to import an already existing docker image

    commands to delete a container

    command to remove all stopped containers, unused networks, build caches, and dangling images?

Here are the commands for the following:

  • To view the running container

      docker ps
  • To run the container under a specific name

      docker run --name <container_id/name> <image_name>
  • To export a docker container

      docker export -o <file_name>.tar <container_id/name>
  • To import an already existing Docker image

      docker import <file_name>.tar <image_name>:tag
  • To delete a container

      docker rm <container_id/name>
  • To remove all the stopped containers, unused networks, build caches, and dangling images

      docker system prune -a

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