Top Docker Interview Questions and Answers in 2021

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With over 8 billion container image downloads by the end of 2017, Docker has turned out to be a big success. And since then, the increasing demand for Docker resulted in more job openings. This blog contains Docker interview questions that cover a range of topics, including:

  • Containers,
  • Docker registry,
  • Fundamental components of the Docker architecture, and
  • The major network drivers supported by the Docker ecosystem.

While there can’t be a single list covering the entire range of Docker container interview questions, you’ll find the most-asked ones here. So let’s roll down the top 50 Docker questions you need to know.

50 Top Docker Interview Questions and Answers

Question: What is Docker?

Answer: Docker is an open-source containerization technology that bundles applications and their dependencies together as containers. It ensures that they work consistently in any environment, be it upgrade, test, or creation. It allows you to automate the deployment of applications in the form of lightweight and portable containers.

It is composed of programs that contain all things required to run a program – code, runtime, framework instruments, framework libraries, and so on. It wraps whatever can be introduced on a worker. This ensures that the product will consistently run the equivalent, paying little heed to its current underlying infrastructure.

Question List down the features of Docker.

Answer:  The main features that make Docker stand out are:

  • Offers an isolated environment that enables systems to run in parallel without interruption.
  • A pocket-friendly way that decreases the deployment time to seconds.
  • All of the deployments are consistent, movable (portable), and scalable because Docker containers are free from environmental limitations.
  • Eases technical configuration and rapid deployment of the application. Undoubtedly, it results in increased productivity.
  • Extremely flexible to perform the updates. Just implement the Docker containers, test them, and roll out new updates.

Question: Explain a Docker Container.

Answer: Containers are the fundamental unit of programming that holds the code and other conditions. It allows applications to run easily, rapidly, and reliably.

A Docker container is made utilizing a Docker image. It is an executable bundle of the product, which holds all that is needed to run an application, such as framework devices, libraries, code, runtime, and settings.

Question: Describe Docker Images.

Answer: A Docker image is a read-only model containing a set of instructions for creating a container that can efficiently run on Docker. It offers a convenient way that will help you to package applications and preconfigured server environments that can be used for private purposes or share publicly with other Docker users.

Question: What is Docker Hub?

Answer: Docker Hub is a vault where all the docker images are stored live. Clients can get information from Docker Hub and use them to compose images and containers. As of now, the Docker Hub is the world’s biggest general store of Docker images.

Question: Please explain Docker Architecture.

Answer: Docker Architecture comprises a Docker Engine, which is a customer worker application with 3 significant segments:

  1. A worker, which is a sort of long-running project called a daemon interaction (the docker command).
  2. A REST API is used to determine interfaces that projects can use to converse with the daemon and give it instructions.
  3. CLI (Command line) uses the Docker REST API to control or communicate with the Docker daemon through scripting or direct CLI commands. Numerous other Docker applications use the fundamental API and CLI.

Question: What is a Dockerfile?

Answer: Docker can assemble images consequently by perusing the guidelines from a file called Dockerfile. A Dockerfile is a book file that contains all the commands a client could approach the command line to gather an image. Utilizing docker construct, clients can make a mechanized form that executes a few command-line guidelines in progression.

Question: Describe Docker Compose in brief.

Answer: Docker Compose is a YAML file that contains insights regarding the administrations, organizations, and volumes for setting up the Docker application. Thus, you can utilize Docker Compose to make separate containers, have them, and get them to speak with one another. Every case will uncover a port for communicating with different containers.

Question: What is Docker Swarm?

Answer: Docker Swarm is a local bunching for Docker. In simple words, it is a container orchestration tool just like Kubernetes but offering tighter integration with Docker containers. Docker Swarm serves the standard Docker API; any apparatus that now speaks with a Docker daemon can utilize Swarm to scale to different hosts straightforwardly.

Question: What is a Docker Namespace?

Answer: A namespace adds a layer of seclusion in containers. Docker gives different namespaces to remain convenient and not influence the fundamental host framework. Few namespace types upheld by Docker are PID, Mount, IPC, User, and Network.

Question: Mention some of the most important and advanced docker commands.


  • docker info – Run this command for information.
  • docker pull – Use this to download an image.
  • docker stats – Get the container information.
  • Docker images – Lists down all the images downloaded.

Question: Mention the steps for the Docker Container life cycle.

Answer: The 3 main steps in the Docker Container lifecycle are mentioned below:

  1. Fabricate
  2. Pull
  3. Run

Question; How to run multiple containers using a single service?

Answer: By utilizing docker-compose, you can run numerous containers using a single service. All docker-compose files utilize the YAML language.

Question: Explain CNM.

Answer: CNM represents the Container Networking Model. It is a norm from Docker that frames the premise of container organizing in a Docker environment.

Question: Does Docker offer help for IPV6?

Answer: Indeed, Docker offers help for IPv6. IPv6 organizing is upheld just on Docker daemons running on Linux. Notwithstanding, if you need to empower IPv6 support in the Docker daemon, you need to modify /etc/docker/daemon.json and set the ipv6 key to valid.

Question: Explain how the default logging driver under Docker can be configured.

Answer: To configure the Docker daemon from default to a particular logging driver, you need to set the log-driver estimation to the logging drive’s name, the daemon.json.file.

Question: List some Docker Commands and their functionalities.

Answer: A few of the most commonly-used Docker commands along with their purposes are mentioned below:

  • Construct – Fabricates a Docker image file.
  • Submit – Makes another image from a Docker container.
  • Make – Makes another container.
  • Docker – Dispatches the Docker daemon.
  • Execute – Executes a Container.

Question: Explain Docker object labels.

Answer: Labels are used for applying metadata to Docker articles. Containers, images, neighborhood daemons, organizations, volumes, and hubs are some examples of Docker object labels.

Question: How to find stored Docker volumes?

Answer: By using the /var/lib/docker/volumes command.

Question: What is Hypervisor?

Answer: A hypervisor makes virtualization conceivable. Also known as a Virtual Machine Monitor, it isolates the host framework and dispenses the assets to each partitioned virtual environment. You can fundamentally have multiple OSs on a solitary host framework. There are two kinds of Hypervisors:

  • Type 1: Often called as Native Hypervisor or Bare metal Hypervisor, it runs straightforwardly on the basic host framework. It has direct admittance to your host’s framework equipment and doesn’t need a base worker working framework.
  • Type 2: This sort of Hypervisor utilizes the fundamental host working framework. It’s likewise called Hosted Hypervisor.

Question: When is the .dockerignore file utilized?

Answer: A regular Dockerfile contains at least one copy command to duplicate documents from the designer machine to the docker image, which has become part of the container.

While reproducing organizers to a docker image, it is conceivable that some undesirable files are likewise replicated to the image. This may make an image cumbersome and consequently, cause execution issues in the container.

To evade this, a document named .dockerignore is created alongside the Dockerfile in a similar registry. This file is utilized to list all the documents and indexes required to be barred while replicating organizers onto the image.

It contains an example, and none of the papers coordinating it are added to the image. This helps abstain from pointlessly sending enormous or small documents and registries to the daemon and possibly adding them to images.

Question: How to check the versions of Docker Client and Server?

Answer: The following command will give you all the data you require:

$ docker adaptation

Question: How to make a container from a Docker image?

Answer: To make a container, you pull an image from the Docker archive and run it utilizing the command:

$ Docker run - it - d <image_name>

Question: What’s associated with scaling a Docker Container?

Answer: Docker Container can be scaled to any level required. Because of Docker’s adaptability, you can have from a couple hundred to two or three thousand to millions of containers. Along with making every container consistent, it unconstrains them to the necessary memory and OS.

Question: What is Docker framework prune?

Answer: Docker framework prune is a command that removes all stopped containers, unused networks, build caches and dangling images. Prune is one of the most useful and robust commands in Docker. The syntax is:

$ docker system prune

Question: Can someone utilize JSON rather than YAML to create documents in Docker?

Answer: You can utilize JSON rather than YAML for creating a document. To utilize JSON files with form, indicate the JSON filename to use:

$ docker-create - f docker-compose.json up

Question: How far do Docker Containers scale? Are there any prerequisites?

Answer: Enormous web organizations like Google and Twitter, and platform providers, such as Heroku and dotCloud, all run on Container innovation. Containers can be scaled to many thousands that can run in parallel.

Discussing prerequisites, containers require the memory and the OS on all occasions and an approach to utilize this memory effectively when scaled.

Question: Name the platforms on which Docker runs.

Answer: Docker runs on various Linux distributions:

  • Ubuntu 12.04, 13.04
  • Fedora 19/20+
  • RHEL 6.5+
  • CentOS 6+
  • Gentoo
  • ArchLinux
  • openSUSE 12.3+
  • Core 3.0+

It can likewise be utilized underway with Cloud stages with the accompanying administrations:

  • Amazon EC2
  • Amazon ECS
  • Google Compute Engine
  • Microsoft Azure
  • Rackspace

Question: Is there an approach to identify the status of a Docker Container?

Answer: 6 potential stages at which a Docker container can be found at a given instant:

  1. Created,
  2. Running,
  3. Paused,
  4. Restarting,
  5. Exited, and
  6. Dead.

Utilize the following command to check for docker state at some random point:

$ docker ps

The above command files down only the running containers by default. To search for all cases, utilize the following command:

$ docker ps - a

Question: Would you be able to eliminate a stopped container from Docker?

Answer: The appropriate response is no. You can’t eliminate a stopped container. The Docker container must be in the halted state before getting eliminated.

Question: Could a container restart without help from anyone else?

Answer: No, it’s impractical for a container to restart without help from anyone else. As a matter of course, the flag- restart is set to false.

Question: Is it better to straightforwardly eliminate the Container utilizing the rm command or stop the Container followed by removing the Container?

Answer: It’s in every case better to stop the Container and afterward eliminate it via the remove command.

$ docker stop <coontainer_id> 
$ docker rm - f <container_id>

Halting the container and afterward eliminating it will permit imparting SIG_HUP sign to beneficiaries. This will guarantee that all the containers will have sufficient opportunity to tidy up their errands. This technique is viewed as a decent work on, keeping away from undesirable blunders.

Question: What number of containers can run per host?

Answer: There can be unlimited numbers of containers. Docker doesn’t put any limitations on it. In any case, you need to consider that each container needs extra space, CPU, and memory, and many other requirements. You, likewise, need to consider the application size.

Question: Is it a decent practice to run stateful applications on Docker?

Answer: The idea driving stateful applications is that they store their information onto the nearby document framework. It would help if you chose to move the application to another machine as recovering information gets difficult. Otherwise, you must not run stateful applications on Docker.

Question: How might you screen Docker underway?

Answer: Docker gives functionalities like docker details and docker occasions to screen Docker underway. Its fact provides CPU and memory utilization of the containers, and occurrences offer data on the docker daemon exercises.

Question: Is it a decent practice to run Docker make underway?

Answer: Indeed, utilizing Docker make underway is the best down-to-earth use of Docker create. When you characterize applications with form, you can utilize this create definition in different creation stages like CI, arranging, testing, and so on.

Question: What changes occur in the Docker compose file while moving it to production?

Answer: These are the accompanying changes you need to make to your compose file before moving your application to the creation environment:

  • Eliminate volume ties, so the code stays inside the Container and can’t be changed outside the Container.
  • Binding to various ports on the host.
  • Indicate a restart strategy.
  • Add additional administrations like log aggregator.

Question: Is it feasible for the cloud to overwhelm the utilization of containerization?

Answer: In this sort of inquiry, you can offer your input. For instance, according to my agreement, albeit the cloud is a decent contender, it can’t supplant containerization. Most organizations are utilizing cloud and containerization pairs to get the best out of the two advancements.

Question: What are the different states of the Docker Container?

Answer: The various states of the Docker Container are:

  • Created – An inactive container that is recently created.
  • Restarting – Container in the process of getting restarted.
  • Running – A container that is in the running condition.
  • Paused – The processes are paused.
  • Exited – A container that has run and completed.
  • Dead – Daemon tried and failed to stop.

Question: Explain the Container orchestration and why we need to utilize it?

Answer: Container orchestration helps in dealing with the containers running in a dynamic and huge environment. They handle and mechanize the accompanying undertakings:

  • Provisioning and sending of containers.
  • Burden adjusting.
  • Assignment of assets between containers.
  • Checking the health of containers and hosts.
  • Scaling of the containers.
  • Switching containers from one host to another on the availability or lack of resources.

Question: How to login into the Docker store?

Answer: You can utilize the following command to login into

$ docker login

You’ll be given a username and secret key. Embed those, and congrats, you’re signed in.

Question: How to utilize a base image and make adjustments or customize it?

Answer: It’s one straightforward command to pull an image from the docker center point:

$ docker pull <image_name>

Question: How to make a Docker Container from an image?

Answer: Pull an image from the Docker store with the below command and run it to make a container:

$ docker run - it - d <image_name>

Question: How would you list all the running Containers?

Answer: The following command files down all the running Containers:

$ docker ps

Question: How would you access a running Container?

Answer: Use the following command to get a running Container. The executive command allows you to get inside a Container and work with it:

$ docker executive – it <container id> slam

Question: How to begin, pause and murder a Container?

Answer: Command to begin a Docker Container:

$ docker start <container_id>

Use the following command to halt a running Container:

$ docker stop <container_id>

Murder a Container with the below-mentioned command:

$ docker murder <container_id>

Question: Would you be able to utilize a Container, alter it, and update it?

Answer: You can utilize a container, alter it and update it. This sounds confounding; however, it’s, in reality, only one command.

$ docker submit <conatainer id> <username/imagename>

Question: How to push an image to the Docker center?

Answer: Use the following command to push an image to a Docker center:

$ docker push <username/image name>

Question: Explain the memory-trade banner?

Answer:  A memory-trade banner is a modifier banner that permits a Container to compose an overabundance of memory necessities into a circle when it has utilized all the accessible RAM. For instance, if the memory = “400m” and memory-trade = “1g”, at that point, the container will utilize 400m of memory and trade 600m (1g-400m).

Question: Explain the difference between ‘Add’ and ‘Copy’ in Dockerfile.

Answer:  The ‘Add’ guidance is utilized to duplicate files from fabricating settings to the image. Besides regular papers, it also permits URL and document (tar, gzip, and so forth) files as the <source> boundary.

When a URL is given, a document is downloaded from the URL and replicated to the <destination>. It consequently unloads packed files, if the <source> contention is a nearby file in a perceived pressure design (tar, gzip, bzip2, and so on).

Though ‘copy’ makes exact duplicate files and organizes from the incorporated setting into the Container, it doesn’t uphold URLs or give any unique treatment to files. Anything that you need to copy into the Container should be available in the nearby form setting.


Docker is a software development platform that runs software packages called “containers”. A container packages up all the dependencies of an application to make it run quickly and reliably from one computing environment to another.

With the companies adopting Docker at an exponential rate, the vacancies available for Docker developers are increasing rapidly. The only thing is you must be thorough with all the components of Docker. Hopefully, the blog must’ve helped you explore the same.

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