Interview › Kubernetes, Docker, Helm & Podman
What is the difference between COPY and ADD?
Kubernetes, Docker, Helm & Podman · Intermediate level
Answer
COPY copies files from the build context into the image. ADD can also extract local tar archives and fetch remote URLs, but I avoid ADD unless I specifically need those extra behaviors because COPY is clearer and safer.
Technical explanation
ADD remote URL behavior is less explicit and less controllable than using curl with checksum validation in a build step.
COPY is preferred for predictable builds and clearer code review.
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: compare COPY and ADD with a tar archive in a lab build.
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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