Interview › Kubernetes, Docker, Helm & Podman
What is a container image, and what are layers?
Kubernetes, Docker, Helm & Podman · Intermediate level
Answer
A container image is an immutable template made of layers plus metadata. Layers are content-addressed filesystem changes, which makes distribution and caching efficient because unchanged layers can be reused.
Technical explanation
Each Dockerfile instruction can create a layer, and layers are reused by digest when unchanged.
Image metadata includes config such as entrypoint, command, exposed ports, env vars, labels, and user.
Container image quality affects supply chain, startup time, vulnerability surface, rollout reliability, and debugging workflows.
Prefer reproducible builds: pinned dependencies, small build context, deterministic Dockerfile order, non-root runtime, and immutable image references.
Understand the runtime boundary: an image is not a VM, and container isolation depends on kernel, namespaces, cgroups, capabilities, seccomp, and mounts.
Hands-on example
1. Create a tiny sample app and Dockerfile for this exercise: build an image and inspect layers with docker history or podman history.
2. Build and inspect it with docker build or podman build, docker history, image inspect, and a vulnerability or size scan if available.
3. Run it locally with explicit env vars, ports, user, volumes, and signal tests depending on the question.
4. Convert the final runtime assumptions into Kubernetes fields such as image, command, args, ports, securityContext, probes, and volumeMounts.
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More Kubernetes, Docker, Helm & Podman interview questions
- What is Kubernetes, and what problem does it solve over running containers manually?
- Explain the Kubernetes control plane components (API server, etcd, scheduler, controller manager).
- What runs on a worker node (kubelet, kube-proxy, container runtime)?
- What is a Pod, and why does Kubernetes schedule Pods rather than containers?
- What is the difference between a Pod, a ReplicaSet, and a Deployment?
- How does a Deployment perform a rolling update, and how do maxSurge and maxUnavailable work?
- How do you roll back a Deployment, and how does Kubernetes track revisions?
- What is a Service, and what are the types (ClusterIP, NodePort, LoadBalancer, ExternalName)?