Question 1

What is a Dockerfile and why do I need one?

Accepted Answer

A Dockerfile is a text script containing instructions for building a Docker container image. It automates the process of packaging your application with its runtime, dependencies, and configuration. Docker containers are portable — they run identically on any machine with Docker installed, eliminating the 'it works on my machine' problem.

Question 2

What is a multi-stage build and should I use it?

Accepted Answer

A multi-stage build uses multiple FROM instructions in a single Dockerfile. The first stage builds your application (including compilers, build tools, dev dependencies), and the second stage copies only the compiled output into a minimal runtime image. This dramatically reduces final image size — a Node.js app might go from 1GB to 100MB.

Question 3

What should I put in .dockerignore?

Accepted Answer

The .dockerignore file works like .gitignore but for Docker build context. Include: node_modules (reinstalled in container), .git directory, .env files (use environment variables at runtime instead), build artifacts, logs, and test files. Reducing the build context speeds up builds and avoids accidentally copying sensitive files.

Question 4

How do I choose the right base image?

Accepted Answer

Prefer official images from Docker Hub (node:20-alpine, python:3.11-slim). Alpine variants are smaller but may lack some tools. 'slim' Debian variants balance size and compatibility. For production, use specific version tags (node:20.10-alpine3.19) rather than 'latest' to ensure reproducible builds.

Question 5

How do I pass environment variables and secrets to containers?

Accepted Answer

Never hardcode secrets in Dockerfiles — they become part of the image layers and are visible in image history. Use ENV for non-sensitive defaults, and pass secrets at runtime via -e flags or Docker secrets. For development, use .env files with docker-compose. For production, use a secrets manager like AWS Secrets Manager or HashiCorp Vault.

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