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Docker is a popular open-source software platform that simplifies the process of creating, managing, running, and distributing applications. It uses containers to package applications along with their dependencies. Docker dominates the market. Most of the top cloud and IT companies have adopted Docker to streamline their application development workflows. The demand for applicants with Docker experience is high.
Cracking your Docker interview is the key to landing one of these highly coveted roles. We’ve gathered the top 40 Docker interview questions to help you prepare for your Docker interview. This Docker tutorial includes both questions and answers. Let’s get started!
We’ll cover:
Docker containers create an abstraction at the application layer and package applications together with all of their dependencies. This allows us to deploy applications quickly and reliably. Containers don’t require us to install a different operating system. Instead, they use the underlying system’s CPU and memory to perform tasks. This means that any containerized application can run on any platform regardless of the underlying operating system. We can also think of containers as runtime instances of Docker images.
A Dockerfile is a text file that contains all of the commands that we need to run to build a Docker image. Docker uses the instructions in the Dockerfile to automatically build images. We can use docker build to create automated builds that execute multiple command-line instructions in sequential order.
To create a container from an image, we pull out the image that we want from the Docker repository and create a container. We can use the following command:
$ docker run -it -d <image_name>
Yes, we can use a JSON file instead of a YAML file for the Docker Compose file. To use JSON, we need to specify the filename like this:
$ docker-compose -f docker-compose.json up
Docker Swarm is a container orchestration tool that allows us to manage multiple containers across different host machines. With Swarm, we can turn multiple Docker hosts into a single host for easy monitoring and management.
We can pull an image from Docker Hub onto our local system using the following Docker command:
$ docker pull <image_name>
To start a Docker container, use the following command:
$ docker start <container_id>
To stop a Docker container, use the following command:
$ docker stop <container_id>
To kill a Docker container, use the following command:
$ docker kill <container_id>
Docker runs on the following Linux distributions:
Docker can also be used in production with these cloud services:
Tip: We always recommend engaging in some company research prior to your interview. To prepare for this particular question, find out how to company uses Docker and include the platform they use in your answer.
The three architectural components include Docker Client, Host, and Registry.
Docker Client: This component executes build and run operations to communicate with the Docker Host.
Docker Host: This component holds the Docker Daemon, Docker images, and Docker containers. The daemon sets up a connection to the Docker Registry.
Docker Registry: This component stores Docker images. It can be a public registry, such as Docker Hub or Docker Cloud, or a private registry.
Virtualization
Virtualization helps us run and host multiple operating systems on a single physical server. In virtualization, hypervisors give a virtual machine to the guest operating system. The VMs form an abstraction of the hardware layer so each VM on the host can act as a physical machine.
Containerization
Containerization provides us with an isolated environment for running our applications. We can deploy multiple applications using the same operating system on a single server or VM. Containers form an abstraction of the application layer, so each container represents a different application.
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A hypervisor, or virtual machine monitor, is software that helps us create and run virtual machines. It enables us to use a single host computer to support multiple guest virtual machines. It does this by dividing the system resources of the host and allocating them to the installed guest environments. Multiple operating systems can be installed on a single host operating system. There are two kinds of hypervisors:
Native: Native hypervisors, or bare-metal hypervisors, run directly on the underlying host system. It gives us direct access to the hardware of the host system and doesn’t require a base server operating system.
Hosted: Hosted hypervisors use the underlying host operating system.
In order to create an image with our outlined specifications, we need to build a Dockerfile. To build a Dockerfile, we can use the docker build command:
$ docker build <path to dockerfile>
To push a new image to the Docker Registry, we can use the docker push command:
$ docker push myorg/img
Docker Engine is an open-source containerization technology that we can use to build and containerize our applications. Docker Engine is supported by the following components:
To access a running container, we can use the following command:
$ docker exec -it <container_id> bash
To list all of the running containers, we can use the following command:
$ docker ps
Docker containers go through the following stages:
Docker object labels are key-value pairs that are stored as strings. They enable us to add metadata to Docker objects such as containers, networks, local daemons, images, Swarm nodes, and services.
Docker Compose doesn’t wait for containers to be ready before moving forward with the next container. In order to control our order of execution, we can use the “depends on” condition, depends_on. Here’s an example of it being used in a docker-compose.yml file:
version: "2.4"services:backend:build: .depends_on:- dbdb:image: postgres
The docker-compose up command will start and run the services in the dependency order that we specify.
The docker create command creates a writable container layer over a specified image and prepares that image for running the specified command.
docker service command do?docker save and docker load commands?docker system prune command do?Answer: Multi-stage builds let you use multiple FROM statements in one Dockerfile so you can build artifacts in a “builder” stage and copy only the final outputs into a tiny runtime image. This reduces image size, surface area, and build time.
Example:
FROM golang:1.22 AS buildWORKDIR /srcCOPY . .RUN go build -o appFROM gcr.io/distroless/baseCOPY --from=build /src/app /appENTRYPOINT ["/app"]
Answer: COPY simply copies local files into the image. ADD also supports remote URLs and automatic extraction of local tar archives. Best practice: prefer COPY for clarity and reproducibility; use ADD only when you explicitly need its extra behaviors.
Answer: The build context is the directory sent to the Docker daemon during docker build. A large context makes builds slow and images bloated. Use .dockerignore (similar to .gitignore) to exclude files like node_modules, .git, and test artifacts so they aren’t sent to the daemon or accidentally copied into images.
Answer: HEALTHCHECK lets Docker periodically test container health. If the command exits non-zero, status becomes unhealthy, which can be used by orchestrators for restarts or routing.
Example:
HEALTHCHECK --interval=30s --timeout=3s \CMD curl -f http://localhost:8080/health || exit 1
Check with docker inspect to see health state.
Answer:
Volumes: Managed by Docker; stored in Docker’s area. Portable, good for persistent data across container lifecycles, easy to back up and share between containers.
Bind mounts: Map a host path into the container. Useful for local dev (live code reload), but tightly couples container to host filesystem layout and permissions.
Answer: Use runtime flags to prevent noisy-neighbor problems and improve reliability:
CPU: --cpus=2 or --cpuset-cpus="0,1"
Memory: --memory=512m --memory-swap=1g
PIDs: --pids-limit=200
These limits are important in multi-tenant hosts and CI runners.
Answer: Common policies:
no (default): never restart.
on-failure[:max-retries]: restart only if exit code ≠ 0.
always: always restart, even after daemon restarts.
unless-stopped: like always but won’t restart if the container was manually stopped.
Use on-failure for batch jobs; unless-stopped/always for long-running services.
Answer: Docker captures container stdout/stderr and routes it via a logging driver (e.g., json-file default, journald, syslog, gelf, fluentd). Configure per-container with --log-driver and options like rotation (--log-opt max-size=10m --log-opt max-file=3). Centralized drivers help ship logs off-host for analysis and retention.
Answer: Avoid baking secrets into images or environment variables. Options include:
Docker Swarm secrets: docker secret create and consume via /run/secrets/....
External secret stores: mount at runtime or fetch on start (e.g., cloud KMS, Vault).
Build-time: never pass secrets in COPY; use build args only for non-sensitive values. For local dev, prefer bind mounts with strict permissions.
Answer:
Run as a non-root user: in Dockerfile USER appuser (create it first).
Minimize image size: use distroless/alpine or multi-stage builds.
Drop capabilities: --cap-drop=ALL --cap-add=NET_BIND_SERVICE (only what you need).
Read-only filesystem: --read-only and mount writable dirs as tmpfs if required.
Pin versions and digests: avoid latest; use immutable tags or content digests.
Scan images: integrate image scanning in CI to catch vulnerabilities early.
Congrats! You made it to the end. Preparing for your Docker interview will take time, so be patient with the process. Be prepared to spend a lot of time studying and preparing. There’s still more to learn about Docker. Some recommended topics to cover next include:
To get started learning these concepts and a lot more, check out Educative’s learning path DevOps for Developers. In this curated learning path, you’ll get hands-on practice with Docker and Kubernetes. By the end, you’ll have cutting-edge stills and hands-on experience so you can excel in your DevOps role.
Happy learning!
