Question 1

What is a Dockerfile?

Accepted Answer

A Dockerfile is a text file containing instructions to build a Docker image. Each instruction creates a layer in the image. Common instructions: FROM (base image), RUN (execute commands), COPY (add files), EXPOSE (port), CMD (start command). Docker reads the Dockerfile and builds an image with docker build command.

Question 2

What is multi-stage Docker build?

Accepted Answer

Multi-stage builds use multiple FROM statements in one Dockerfile. Each FROM starts a new build stage. You can COPY artifacts from one stage to another, discarding intermediate files. Result: smaller production images without build tools, dependencies, or source code. Example: build stage with Go compiler, final stage with only the compiled binary.

Question 3

Why use Alpine Linux in Docker?

Accepted Answer

Alpine Linux is a minimal Docker base image (~5MB vs ~100MB for Debian/Ubuntu). Based on musl libc and busybox. Smaller images mean faster downloads, less storage, fewer vulnerabilities to scan. Use node:18-alpine, python:3.11-slim, golang:1.21-alpine. Note: musl libc may cause issues with some npm packages compiled for glibc.

Question 4

How do I optimize Dockerfile layer caching?

Accepted Answer

Docker caches each layer. If a layer changes, all subsequent layers rebuild. Optimization: 1) Put frequently changing instructions (COPY source code) last. 2) Put stable instructions (FROM, RUN install dependencies) first. 3) Combine related RUN commands with &&. 4) Use .dockerignore to exclude unnecessary files. This speeds up rebuilds significantly.

Question 5

What is Docker HEALTHCHECK?

Accepted Answer

HEALTHCHECK instruction tells Docker how to test container health. Docker runs the command periodically (default 30s). If exit code is 0, container is healthy. If non-zero, container is unhealthy. Kubernetes and Docker Swarm use health status for automatic restarts. Example: HEALTHCHECK CMD curl -f http://localhost:3000/ || exit 1

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