Top 65 Docker Interview Questions And Answers

Question 1

What is Docker?

Accepted Answer

* Docker is a containerization platform which packages your application and all its dependencies together in the form of containers so as to ensure that your application works seamlessly in any environment be it development or test or production.
* Docker containers, wrap a piece of software in a complete filesystem that contains everything needed to run: code, runtime, system tools, system libraries etc. anything that can be installed on a server.
* This guarantees that the software will always run the same, regardless of its environment.

Question 2

What is the difference between ‘docker run’ and ‘docker create’?

Accepted Answer

The primary difference is that using **‘docker create’** creates a container in a stopped state.

**Bonus point:** You can use **‘docker create’** and store an outputed container ID for later use. The best way to do it is to use **‘docker run’** with -**\-cidfile FILE\_NAME** as running it again won’t allow to overwrite the file. A good practice is to keep well ogranised directory structure: /containers/web/server1/ws.cid containers/web/server3/ws.cid

Question 3

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

Accepted Answer

Although `ADD` and `COPY` are functionally similar, generally speaking, `COPY` is preferred.

That’s because it’s more transparent than ADD. COPY only supports the basic copying of local files into the container, while ADD has some features (like local-only tar extraction and remote URL support) that are not immediately obvious. Consequently, the best use for ADD is local tar file auto-extraction into the image, as in ADD rootfs.tar.xz /.

Question 4

What are the various states that a Docker container can be in at any given point in time?

Accepted Answer

There are four states that a Docker container can be in, at any given point in time. Those states are as given as follows:

* Running
* Paused
* Restarting
* Exited

Question 5

When would you use ‘docker kill’ or ‘docker rm -f’?

Accepted Answer

If you must stop the container really quickly… (someone pushed something to production on Friday evening?… ;) )

Question 6

Is there a way to identify the status of a Docker container?

Accepted Answer

We can identify the status of a Docker container by running the command

```sh
docker ps –a
```

which will in turn list down all the available docker containers with its corresponding statuses on the host. From there we can easily identify the container of interest to check its status correspondingly.

Question 7

What does _Containerization_ mean?

Accepted Answer

*Containerisation* is a type of *virtualization* strategy that emerged as an alternative to traditional hypervisor-based virtualization.

In containerization, the operating system is shared by the different containers rather than cloned for each virtual machine. For example Docker provides a container virtualization platform that serves as a good alternative to hypervisor-based arrangements.

Question 8

What’s the difference between a repository and a registry?

Accepted Answer

* **Docker registry** is a service for hosting and distributing images (the default one is the Docker Hub). 
* **Docker repository** is a collection of related Docker images (the same name but with different tags).

Question 9

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

Accepted Answer

Both `CMD` and `ENTRYPOINT` instructions define what command gets executed when running a container. There are few rules that describe their co-operation.
 
1. Dockerfile should specify at least one of `CMD` or `ENTRYPOINT` commands.
2. `ENTRYPOINT` should be defined when using the container as an executable.
3. `CMD` should be used as a way of defining default arguments for an `ENTRYPOINT` command or for executing an ad-hoc command in a container.
4. `CMD` will be overridden when running the container with alternative argumen

Question 10

What is Docker hub?

Accepted Answer

**Docker hub** is a cloud-based registry service which allows you to link to code repositories, build your images and test them, stores manually pushed images, and links to Docker cloud so you can deploy images to your hosts. It provides a centralized resource for container image discovery, distribution and change management, user and team collaboration, and workflow automation throughout the development pipeline.

Top 65 Docker Interview Questions

What is Docker?

What does Containerization mean?

Can you remove (‘docker rm’) a container that is paused?

Do I lose my data when the Docker container exits?

What is Build Cache in Docker?

What’s the difference between a repository and a registry?

How to build envrionment-agnostic systems with Docker?

How to link containers?

What type of applications - Stateless or Stateful are more suitable for Docker Container?

What is Docker image?

What is Docker container?

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

What is the difference between ‘docker run’ and ‘docker create’?

Is there a way to identify the status of a Docker container?

What are the most common instructions in Dockerfile?

What are the various states that a Docker container can be in at any given point in time?

What is Docker hub?

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

When would you use ‘docker kill’ or ‘docker rm -f’?

What is the difference between a Docker image and a container?

What is Hypervisor?

How can we control the startup order of services in Docker compose?

Explain a use case for Docker

Explain what are some Pods usage patterns?

Which problems does a Container Orchestration solve?

Explain when to use Docker vs Docker Compose vs Docker Swarm vs Kubernetes

What is the preferred way of removing containers - ‘docker rm -f’ or ‘docker stop’ then followed by a ‘docker rm’?

What is the difference between Kubernetes and Docker?

What is the default CPU limit set for a container?

What is the purpose of EXPOSE command in Dockerfile?

What is the difference between “expose” and “publish” in Docker?

Should I use Vagrant or Docker for creating an isolated environment?

How will you monitor Docker in production?

Could you explain what is Emulation?

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

What is virtualisation?

Docker Compose vs. Dockerfile - which is better?

What exactly do you mean by “Dockerized node”? Can this node be on-premises or in the cloud?

What happens if you add more than one CMD instruction to a Dockerfile?

Explain basic Docker usage workflow

What is the difference between Docker Image and Layer?

Can you create containers wihout their own PID namespace?

What is Docker Swarm?

Can you explain dockerfile ONBUILD instruction?

Can you run Docker containers natively on Windows?

Is it good practice to run stateful applications on Docker? What are the scenarios where Docker best fits in?

What are the different kinds of namespaces available in a Container?

Why do we need Kubernetes (and other orchestrators) above containers?

What is the difference between Docker RUN, CMD and ENTRYPOINT?

How is Container different from a Virtual Machine?

What is Paravirtualization?

When you limit the memory for a container, does it reserve (guarantee) the memory?

Is it possible to generate a Dockerfile from an image?

What is an orphant volume and how to remove it?

How virtualization works at low level?

How is Docker different from a virtual machine?

How to use Docker with multiple environments?

Why did Docker jump from version 1.13 to 17.03?

How containers works at low level?

How does Docker run containers in non-Linux systems?

Why Docker compose does not wait for a container to be ready before moving on to start next service in dependency order?

Name some limitations of containers vs VM

Can you explain a relationship between container runtime and container orchestration?

What's the difference between pm2 and pm2-runtime and when to use one?

How do I transfer a Docker image from one machine to another one without using a repository, no matter private or public?

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

What's the difference between `pm2` and `pm2-runtime` and when to use one?