15 Docker Interview Questions: Score That Job

With companies constantly becoming more and more ambitious with their projects, it is just natural that a search for methods to make the processes faster and smoother is always there. Various companies choose different tools to aid them in their work. In turn, this leads to them trying to hire people that already know how to use these tools (or at least be familiar with them). One of the more popular of these tools is called Docker, and in this Docker interview questions tutorial, we’re going to talk about the requirements that you might encounter while trying to get a job as a developer in one of these companies.

We’ll cover both the basic and advanced Docker interview questions and answers. I’ll tell you what to expect and why employers tend to ask certain questions over others. At the end of this guide, you should have a pretty good idea of what to expect in your next Docker-related job interview.

The Basics of Docker

As I’ve mentioned, we’ll start the tutorial with some of the more basic, introductory-level interview questions on Docker. This will help you get a general idea of what types of questions to expect during your job interview, and what are the best ways of answering them. Once you feel comfortable enough, we’ll talk a bit more about the experienced-level Docker interview questions.

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Question 1: What is Docker?

You’ve probably expected this, but this is the question that you are bound to get asked at the very beginning of your interview - honestly, it will probably be the first question!

There are a few reasons why your employers might ask this question. First and foremost, they might want to check if you truly do know what Docker is, or are you just a random person off the street that somehow managed to get into the interview. Secondly, your employers might want to check how well can you define difficult subjects - in other words, if you can simply explain what Docker is.

Sometimes these types of Docker interview questions are aimed to put you off balance. Many people spend hundreds of hours practicing, studying and revising the different aspects of Docker, without even thinking about the actual definition of the program.

So, with that said… What is Docker?

Docker is a tool that is designed to help developers create and maintain their apps in an easy and organized way. The way that this program achieves it is by utilizing what is known as containers. The developer can simply add his app (and all of the other required data) into the container and transport it to its destination. If you can imagine a carrier boat carrying those huge boxes with products in them - that’s exactly the right visual representation of Docker.

All of this is done without a need for an additional operating system - Docker uses the Linux kernel to communicate the packaged app from the developer to its destination. That’s why it’s so popular with companies that utilize the DevOps philosophy.

Question 2: What are ‘containers’?

Easily one of the most important Docker interview questions out there. Since containers are such an important aspect of Docker, you are bound to get asked (perhaps even multiple questions!) about them.

Docker containers are the tools that Docker uses to pack and ship developer’s apps to their target destination. These containers are a highly regarded Docker feature because they can run on any type of machine - in other words, they aren’t OS-exclusive. The universality that these containers provide becomes a valuable tool for developers and programmers alike.

Question 3: What is Docker ‘images’?

On the same level of importance as the previous Docker interview questions, images are used to create the container within Docker. This is done by issuing the “run” command.

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Question 4: Did Docker come up with the ‘container’ technology?

Nope, not at all. Docker is a semi-new project - multiple other development tools offer containers, the same as Docker. This is the type of Docker interview questions that might have a follow-up. And what would be the natural follow-up to a question like this?

You’ve guessed it.

How is Docker better than other tools that use containers, then?

The answer is pretty simple. Docker utilizes the cloud to run its container-related operations - something that not many other development tools use. Using the cloud, Docker becomes much more flexible and adaptable to different scenarios that might come up during the development or shipment processes. This is the main feature of why it is being quite heavily regarded, especially when compared to other container-based developer tools.

Question 5: What is a Dockerfile?

A great example of some of the more basic interview questions on Docker - and not a difficult one at that!

Remember the comparison I made between Docker and a carrier boat? Well, somebody has to give the boat instructions on what to carry, where to carry it and so on. A Dockerfile is exactly such a set of instructions. Developers provided Docker with such instructions so that the program could do the job correctly, with those specific parameters in mind.

Question 6: Is the ‘container technology’ reliable?

Although not necessarily one of Docker interview questions, your potential employers might want to check if you’re knowledgeable on the topic in general. And a question about the container technology is a great way to do this.

Concerning container reliability, there is no better way to answer this question than by diving direct, specific examples. The biggest companies in the world (such as Google, Amazon, Intel and so on) use and trust the container technology - this should be the best answer for if containers are a reliable technology to use or not.

Question 7: What are the three main types of Docker components?

The Client, the Host, and the Registry.

The client is the component that issues “run” and “build” commands to the host. The host is where all of the containers and images are created. They are then sent to the registry, for execution.

Question 8: Will you lose all of your work if you accidentally exit a container?

Similar Docker interview questions might aim to throw you off balance and make you second-guess your knowledge. However, don’t be discouraged by your answer!

No, you won’t lose any information, data and other parameters if you accidentally exit the Docker container. The only way to lose your progress would be to issue a specific command to delete the container - exiting it won’t do the files within any harm.

Question 9: Can you use any other files for composing instead of the default YAML?

Yes, yes you can. Truth be told, the more popular version to use than YAML is the good-old JSON. And there are no clashings or any other types of unwanted occurrences - you can use it without a problem.

Question 10: What is ‘NameSpaces’ used for?

NameSpaces isolate the Docker containers from other activities or tampering with them.

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Docker Interview Questions - Advanced

Now that you’re somewhat familiar with the basic interview questions on Docker, we can move on to some of the more advanced questions that you might receive during your job interview.

These questions are aimed at testing just how in-depth your knowledge on the topic truly is. The example questions that this tutorial provides should give you an idea of what to expect during the actual job interview - needless to say, with so many different variations of the questions available, it’s impossible to cover all of them in a single tutorial.

Don’t get discouraged, though! If you’ve been using Docker for some time now, even the advanced Docker interview questions shouldn’t be a problem for you. So, put on your thinking cap and let’s jump straight on in!

Question 1: What is the single most important requirement for building a Docker container?

The most important requirement for building a container with Docker is the default image. This default image may vary depending on the code that you’re using. To find out (and access) the default image, you should go to the Docker Hub and search for the specific domain that you need. After you find the image, all that’s left to do is deal with the documentation and that's it - you can create a Docker container!

Question 2: How does Docker manage ‘Dockerized nodes’?

First of all, you could say that this is one of the double Docker interview questions, for if you want to give a thorough answer (or any logical answer at that), you have to know what a “Dockerized node” even is.

A Dockerized node can be any machine that has Docker installed and running. Now, when it comes to management, the answer is pretty simple - Docker can manage both in-house and cloud-based nodes. So, whether the node exists in the area of the main computer running Docker or it’s present on the cloud - it does not matter. Docker will manage it without a problem.

Question 3: What are the main factors that dictate the number of containers you can run?

There isn’t a clearly-defined limit of containers that you run with Docker. However, that being said, the limitations come in when we start talking about hardware.

Two factors might limit the number of containers that you can run - the size of your app and your CPU strength. If your application isn’t ginormous and you have a never-ending supply of CPU power, you could probably run a huge amount of Docker container simultaneously.

Question 4: How is Docker different from Hypervisor?

Again, one of the Docker interview questions that require you to have some more extensive knowledge of development tools other than Docker. Answering this question thoroughly, you show competence in the general field of development, and not only things relating to Docker (which is a good thing!).

It all boils down to one, simple thing - Hypervisor requires you to have extensive hardware to function properly, while Docker runs on the actual operating system. This allows Docker to be exceptionally fast and perform tasks in a fluid manner - something that Hypervisor tends to lack.

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Question 5: What is the best place to find decent examples of ‘compose files’?

Believe it or not, this question might have some underlying tones to it.

Most of the high-key companies that require Docker experts (or wannabe-experts - it depends) use a specific tool to manage their internal workings. That tool is called GitHub.

Other than all of the main functions that it performs, it is also a great place to find the before-mentioned compose files for Docker containers. It is recommended that you state GitHub as the main answer to this question - chances are, that is exactly what your employers are looking for.

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Conclusions

So - that’s it! We have reached the end of the “Docker Interview Questions and Answers” tutorial! At this point, you should have at least some sort of an idea of what to expect from your job interview.

When it comes to the basic questions, you should look at them as they are - basic questions. Don’t stress, try to elaborate with your answers where possible and just relax - you’ve got this! As for the advanced questions, you might start seeing a pattern.

If you’re unlucky (or lucky?) enough to attend a few Docker-related job interviews, you might notice that most of the interviewers ask very similar questions. Sure, the variables might differ, but the questions do more or less sound the same. Once you notice this, you will see that there is truly no need to analyze Docker from the ground up and back - all that you need is to know the answers to those specific questions. Chances are that the Docker interview questions and answers provided in this tutorial are going to match the ones that you’ll get during your interview.

Why is this the case? Well, a lot of employers are looking for people who can perform the same (or at least similar) tasks with Docker. You can’t force the program to perform miracles - it is what it is. That’s why even you get unlucky with the first interview - don’t give up! You’ll do great in the next one!

I hope this tutorial was useful for you, and that you’ll do great in your job interview!