How (and why) to build minimal and reproducible Docker images with Nix

This repository contains a presentation and examples for building Docker images with Nix, presented at Nix Bern on June 26, 2025.

Prerequisites

• Nix package manager with flakes enabled
• Docker (for running the built images)

Running the Presentation

1. Enter the development shell:

   nix develop

2. Start the presentation:

   presenterm -X -x presentation.md

Examples

Simple Hello World Container

Build a minimal container that runs the hello program:

# Build the image
nix-build hello.nix

# Load into Docker
docker load < result

# Run it
docker run hello-nix:latest

Multi-layered SPA Container

The spa/ directory contains a React SPA example that demonstrates:

• Building a Node.js application with Nix
• Creating a layered Docker image
• Serving static files with Caddy
• Alternative container.nix file to build a container to serve images using nginx

cd spa

# Build the Docker image
nix build .#docker

# Load into Docker
docker load < result

# Run the container
docker run -p 8080:80 spa-nix:latest

You can then visit http://localhost:8080 to see the running application.

Alternatively you can build the nginx based container image:

cd spa

# Build the Docker image
nix build -f container.nix

# Load into Docker
docker load < result

# Run the container
docker run -p 8080:80 spa-nix:latest

You can compare this image with the container image you can build using Dockerfile.nginx:

cd spa
docker build -t spa-docker -f Dockerfile.nginx .

Exploring the Images

Use dive to explore the layer structure:

# Examine the layers
dive hello-nix:latest
dive spa-nix:latest

Key Benefits of Nix for Container Images

• Reproducible: Same inputs always produce the same outputs
• Minimal: Only include what your application actually needs
• Cacheable: Efficient layer sharing across different images
• Transparent: Complete dependency tracking and SBOM generation

Files Structure

• presentation.md - The main presentation slides
• hello.nix - Simple hello world container example
• spa/ - React SPA example with Dockerfile comparison
• flake.nix - Development environment setup
• links.md - Useful resources and links

Development

The flake provides all necessary tools for the presentation:

• presenterm - For presenting the slides
• crane - For examining container manifests
• jq - For JSON processing
• qrencode - For generating QR codes
• dive - For exploring container layers
• sbomnix - For generating Software Bill of Materials