eliferjunior Nixpacks — App Source to Docker Image
Overview
Nixpacks, the build system created by Railway that automatically detects your application's language and framework, installs dependencies, and produces optimized Docker images — all without writing a Dockerfile. Helps developers configure Nixpacks for custom build steps, multi-language projects, and CI/CD integration.
Instructions
Basic Usage
Build any application into a Docker image:
# Install Nixpacks
curl -sSL https://nixpacks.com/install.sh | bash
# Build an image (auto-detects language and framework)
nixpacks build ./my-node-app -n my-app
# Detects: Node.js + Next.js → installs Node, npm ci, npm run build
nixpacks build ./my-python-api -n my-api
# Detects: Python + FastAPI → installs Python, pip install, uvicorn start
nixpacks build ./my-rust-service -n my-service
# Detects: Rust + Cargo → installs Rust, cargo build --release
# Run the built image
docker run -p 3000:3000 my-app
# Generate a Dockerfile without building (inspect what Nixpacks would do)
nixpacks plan ./my-node-app
# Shows: detected providers, install/build/start commands, Nix packages
# Generate Dockerfile for manual editing
nixpacks build ./my-node-app --out . --name my-app
# Creates .nixpacks/Dockerfile that you can customize
Configuration
Override auto-detected settings:
# nixpacks.toml — Custom build configuration
[phases.setup]
# Additional system packages (via Nix)
nixPkgs = ["ffmpeg", "imagemagick", "poppler_utils"]
aptPkgs = ["libvips-dev"] # Debian packages (fallback)
[phases.install]
cmds = ["npm ci --production=false"] # Override install command
[phases.build]
cmds = [
"npx prisma generate", # Generate Prisma client
"npm run build", # Build the application
]
[start]
cmd = "node dist/server.js" # Override start command
# Environment variables available during build
[variables]
NODE_ENV = "production"
NEXT_TELEMETRY_DISABLED = "1"
# nixpacks.toml — Python project with system dependencies
[phases.setup]
nixPkgs = ["postgresql"] # For psycopg2 compilation
pythonVersion = "3.12"
[phases.install]
cmds = ["pip install -r requirements.txt"]
[phases.build]
cmds = [
"python manage.py collectstatic --noinput",
"python manage.py migrate --check",
]
[start]
cmd = "gunicorn myapp.wsgi:application --bind 0.0.0.0:$PORT --workers 4"
Multi-Language Projects
Handle monorepos and multi-language apps:
# nixpacks.toml — Monorepo with frontend + backend
[phases.setup]
nixPkgs = ["nodejs-20_x", "python312"]
[phases.install]
cmds = [
"cd frontend && npm ci",
"cd backend && pip install -r requirements.txt",
]
[phases.build]
cmds = [
"cd frontend && npm run build",
"cp -r frontend/dist backend/static",
"cd backend && python manage.py collectstatic --noinput",
]
[start]
cmd = "cd backend && gunicorn app:app --bind 0.0.0.0:$PORT"
CI/CD Integration
Use Nixpacks in GitHub Actions:
# .github/workflows/deploy.yml — Build and push with Nixpacks
name: Deploy
on:
push:
branches: [main]
jobs:
build-and-deploy:
runs-on: ubuntu-latest
steps:
- uses: actions/checkout@v4
- name: Install Nixpacks
run: curl -sSL https://nixpacks.com/install.sh | bash
- name: Build image
run: |
nixpacks build . \
--name ghcr.io/${{ github.repository }}:${{ github.sha }} \
--env NODE_ENV=production
- name: Login to GHCR
run: echo "${{ secrets.GITHUB_TOKEN }}" | docker login ghcr.io -u ${{ github.actor }} --password-stdin
- name: Push image
run: |
docker push ghcr.io/${{ github.repository }}:${{ github.sha }}
docker tag ghcr.io/${{ github.repository }}:${{ github.sha }} ghcr.io/${{ github.repository }}:latest
docker push ghcr.io/${{ github.repository }}:latest
Supported Languages
## Auto-Detected Languages and Frameworks
| Language | Detection File | Frameworks |
|-----------|------------------------|--------------------------------------|
| Node.js | package.json | Next.js, Nuxt, Remix, Express, Nest |
| Python | requirements.txt/pyproject.toml | Django, Flask, FastAPI, Streamlit |
| Rust | Cargo.toml | Actix, Axum, Rocket |
| Go | go.mod | Gin, Echo, Fiber, net/http |
| Ruby | Gemfile | Rails, Sinatra |
| PHP | composer.json | Laravel, Symfony |
| Java | pom.xml/build.gradle | Spring Boot, Quarkus |
| Elixir | mix.exs | Phoenix |
| Haskell | stack.yaml | - |
| Zig | build.zig | - |
| Crystal | shard.yml | - |
| Dart | pubspec.yaml | - |
| Swift | Package.swift | Vapor |
| .NET | *.csproj | ASP.NET |
| Static | index.html | - |
Examples
Example 1: Setting up Nixpacks for a microservices project
User request:
I have a Node.js API and a React frontend running in Docker. Set up Nixpacks for monitoring/deployment.
The agent creates the necessary configuration files based on patterns like # Install Nixpacks, sets up the integration with the existing Docker setup, configures appropriate defaults for a Node.js + React stack, and provides verification commands to confirm everything is working.
Example 2: Troubleshooting configuration issues
User request:
Nixpacks is showing errors in our configuration. Here are the logs: [error output]
The agent analyzes the error output, identifies the root cause by cross-referencing with common Nixpacks issues, applies the fix (updating configuration, adjusting resource limits, or correcting syntax), and verifies the resolution with appropriate health checks.
Guidelines
- Start without config — Try
nixpacks build .first; auto-detection handles most projects correctly - Use nixpacks.toml for system deps — Need FFmpeg, ImageMagick, or native libraries? Add them to
nixPkgs - Pin language versions — Specify
pythonVersion = "3.12"or use.nvmrc/.python-versionfiles - Check the plan first — Run
nixpacks plan .to see what Nixpacks will do before building - Cache layers — Nixpacks caches install and build phases; put dependency files (package.json) before source code
- Generate Dockerfile for debugging — Use
--out .to get the generated Dockerfile; inspect and customize if needed - Environment variables in build — Use
[variables]in nixpacks.toml for build-time vars; runtime vars come from your platform - Nix packages over apt — Prefer
nixPkgsoveraptPkgs; Nix packages are more reproducible and better cached