write-dockerfile

Write Dockerfile Skill

Create a production-ready Dockerfile for the current project. Scope boundary: Dockerfile and image concerns only. For local multi-service orchestration (compose.yaml, service dependencies/readiness, env files, volumes, migrations/seeds), use docker-compose-local-setup.

Step 1 — Detect Language and Framework

Read: package.json, go.mod, pyproject.toml, composer.json, or other manifest files. Identify:

• Runtime and version (Node 22, Go 1.26, Python 3.12, PHP 8.3)
• Build process (compile, bundle, install dependencies)
• Entry point / start command

Step 2 — Apply Dockerfile Build and Runtime Best Practices

Multi-stage build: separate build stage from runtime stage to keep the final image small.

Layer caching: copy dependency manifests first, install, then copy source. This caches the dependency install layer and only invalidates it when dependencies change.

Non-root user: run the application as a non-root user in the final stage.

Minimal base image: use official slim/alpine variants for the runtime stage.

Health check: include a HEALTHCHECK instruction when the runtime supports it.

No secrets in the image: all secrets come from environment variables at runtime.

Step 3 — Write the Dockerfile

Structure:

# syntax=docker/dockerfile:1
# Stage 1: Build
FROM {build-image} AS builder
WORKDIR /app
# Copy manifests first (cache layer)
COPY {manifest-files} ./
RUN {install-dependencies}
# Copy source and build
COPY . .
RUN {build-command}

# Stage 2: Runtime
FROM {runtime-image}
WORKDIR /app
RUN addgroup --system app && adduser --system --ingroup app app
# Copy only what is needed to run
COPY --from=builder /app/{output} .
USER app
EXPOSE {port}
HEALTHCHECK --interval=30s --timeout=5s --retries=3 \
  CMD {health-check-command}
CMD ["{start-command}"]

Step 4 — Write .dockerignore

Also create or update .dockerignore to exclude:

• .git/
• node_modules/ (for Node projects)
• test files, coverage reports
• .env files
• *.md documentation files