Originally published on Naveed Ahmed's Engineering Blog.
Docker interviews are no longer only about explaining images and containers.
For experienced DevOps and SRE roles, interviewers may also ask how you would investigate failures, reduce CI build times, or secure container workloads.
I recently put together 50 Docker interview questions organized into six technical areas.
Here are a few practical questions worth mastering.
1. Docker vs Virtual Machines: What's the Real Difference?
Containers use operating-system features such as Linux namespaces and cgroups for isolation and resource management.
Unlike a typical virtual machine, a Linux container shares the host's kernel.
That makes containers useful for packaging and running applications without requiring a full guest OS for each workload.
But containers and VMs offer different isolation boundaries. Sharing a kernel has security implications.
2. What Happens Behind docker run?
A useful interview answer should explain the architecture.
Docker CLI
↓
Docker Daemon
↓
containerd
↓
runc
↓
Linux Container Process
The CLI requests an operation, the daemon manages Docker resources, containerd supports container lifecycle operations, and the OCI runtime creates the container process.
Knowing the components helps when troubleshooting runtime or daemon failures.
3. How Would You Fix Slow Docker Builds?
Imagine a Docker image takes 25 minutes to build in CI even when only one application file changes.
The first place I'd look is the Dockerfile.
A common problem is copying all application files before installing dependencies.
For a Node.js application, a better starting point may be:
FROM node:22-slim
WORKDIR /app
COPY package*.json ./
RUN npm ci
COPY . .
CMD ["node", "server.js"]
This illustrates layer ordering, not a complete production-hardened image.
When the dependency files remain unchanged, Docker can often reuse the dependency-installation layer.
For more demanding workloads, consider BuildKit cache mounts and external cache exports.
4. What Does Docker Exit Code 137 Mean?
Exit code 137 means a process was terminated by signal 9, SIGKILL.
It may indicate an out-of-memory kill, but it can also occur when a process is forcibly killed for another reason.
Start with:
docker inspect <container> \
--format '{{.State.ExitCode}} {{.State.OOMKilled}}'
Then review logs and host evidence:
docker logs --tail 100 <container>
sudo journalctl -k --no-pager | grep -i oom
The important part is confirming the cause before applying a fix.
5. Why Can't Two Containers Resolve Each Other?
For containers on user-defined bridge networks, Docker provides name-based service discovery.
Check the connected networks:
docker inspect <container> \
--format '{{json .NetworkSettings.Networks}}'
Inspect the network:
docker network inspect <network-name>
Verify that both containers belong to the intended network and test DNS from an appropriate diagnostic container.
Don't assume that published host ports are required for container-to-container communication.
6. How Do You Secure Container Builds?
A useful production checklist includes:
- Multi-stage builds to keep build tools out of runtime images
- A non-root runtime user where feasible
- Least-privilege Linux capabilities
- BuildKit secret mounts instead of embedding credentials
- Base-image updates and vulnerability scanning
- Restricted access to the Docker daemon socket
Mounting /var/run/docker.sock into an untrusted container can expose powerful host-level Docker controls.
Rootless mode reduces privilege exposure, but isn't a complete security guarantee.
7. How Does Docker Relate to Kubernetes?
Docker images commonly use OCI-compatible image formats.
Kubernetes no longer requires Dockershim to interact with Docker Engine; modern clusters can use CRI-compatible runtimes such as containerd directly.
That doesn't mean Docker-built images stopped working with Kubernetes.
It means the orchestration runtime integration changed.
How I'd Answer Scenario-Based Questions
I prefer this format:
Symptom → Evidence → Root cause → Safe fix → Prevention
If the interviewer asks about a failed container, don't jump immediately to restarting it.
Explain which evidence you'd collect and what that evidence would tell you.
Read All 50 Docker Interview Questions
My complete guide covers:
- Docker Engine architecture and Linux isolation
- Dockerfile optimization and BuildKit
- Storage, volumes and networking
- Production incidents and exit codes
- Container security and build environments
- Kubernetes and container runtime integration
Read 50 Docker Interview Questions and Answers for 2026
You can also practice more hands-on scenarios on the DevOps Interview Hub.
What's the most difficult Docker issue you've troubleshot in production?
Written by Naveed Ahmed — DevOps & Platform Engineering
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