Dockerfile & Railpack Builds

Every project has a build mode. You do not always need a Dockerfile — Auto and Railpack can build from language manifests (for example package.json, requirements.txt, pyproject.toml).

Build modes

Mode	Behaviour
Auto (default)	Use a repository Dockerfile if present; otherwise Railpack
Dockerfile	Always your Dockerfile — fails loudly if missing
Railpack	Always Railpack, even if a Dockerfile exists

Railpack (the builder behind Railway) detects Node, Python, Go, PHP, Ruby, Java, Rust, and more, then builds with BuildKit.

Extra settings:

• Base directory — monorepo subdirectory to build from
• Dockerfile path — non-root Dockerfile location

Dockerfile tips

When you do ship a Dockerfile, include an EXPOSE directive so Docklift can detect the internal listen port for proxying and optional host-port mapping.

Node.js example

FROM node:20-alpine
WORKDIR /app
COPY package*.json ./
RUN npm install
COPY . .
EXPOSE 3000
CMD ["npm", "start"]

Python Flask example

FROM python:3.11-slim
WORKDIR /app
COPY requirements.txt .
RUN pip install -r requirements.txt
COPY . .
EXPOSE 5000
CMD ["python", "app.py"]

Ports and networking

• The app must listen on 0.0.0.0 inside the container (not only 127.0.0.1)
• Internal port comes from EXPOSE, project settings, or Railpack detection
• Public access is preferably via a custom domain on nginx-proxy
• Host ports (5500–5600) are opt-in — see Port Management

Build-time secrets

Mark env vars as build when the image needs them at build time.

• Plain build vars map to Docker ARG / --build-arg when declared
• BuildKit secret (with build): passed as a Docker secret, not --build-arg. In the Dockerfile:

RUN --mount=type=secret,id=MY_SECRET \
  MY_SECRET="$(cat /run/secrets/MY_SECRET)" && ./build.sh

Avoid baking runtime credentials into layers. Prefer runtime env injection — see Environment Variables.

Multi-Dockerfile layouts

my-project/
├── api/Dockerfile
├── frontend/Dockerfile
└── worker/Dockerfile

Each Dockerfile becomes a service. Deploy still runs at the project level. Details: Deployment.