Docker Image Size Estimator

Smaller images pull faster in CI/CD pipelines, start containers faster (especially in serverless/ephemeral environments), consume less storage in registries (reducing costs), and have a smaller attack surface with fewer packages to patch.

Multi-stage builds use multiple FROM statements. The first stage installs build tools and compiles your app. The second (final) stage copies only the compiled output into a minimal base image. This keeps build dependencies (gcc, node_modules/devDependencies) out of the final image. Example: Go apps can build in golang:alpine and run in scratch.

Default (e.g., node:20) is based on Debian Bookworm with all common tools — largest but most compatible. slim variants remove documentation, man pages, and some utilities, cutting size significantly. alpine variants use Alpine Linux with musl libc — smallest, but may have compatibility issues with packages expecting glibc.

Always combine apt-get update, apt-get install, and apt-get clean in a single RUN command: RUN apt-get update && apt-get install -y pkg && rm -rf /var/lib/apt/lists/*. If you split these into separate RUN commands, Docker caches each layer separately, and the cache files remain in the image.

Similar to .gitignore, .dockerignore lists files that should not be sent to the Docker build context. Always include node_modules, .git, .env, dist, and build artifacts. A missing .dockerignore can cause your COPY . . instruction to include gigabytes of irrelevant files in the build context.