Writing Dockerfiles Agent Skill

Writing Dockerfiles

Create production-grade Dockerfiles with multi-stage builds, security hardening, and language-specific optimizations.

When to Use This Skill

Invoke when:

• "Write a Dockerfile for [Python/Node.js/Go/Rust] application"
• "Optimize this Dockerfile to reduce image size"
• "Use multi-stage build for..."
• "Secure Dockerfile with non-root user"
• "Use distroless base image"
• "Add BuildKit cache mounts"
• "Prevent secrets from leaking in Docker layers"

Quick Decision Framework

Ask three questions to determine the approach:

1. What language?

• Python → See references/python-dockerfiles.md
• Node.js → See references/nodejs-dockerfiles.md
• Go → See references/go-dockerfiles.md
• Rust → See references/rust-dockerfiles.md
• Java → See references/java-dockerfiles.md

2. Is security critical?

• YES → Use distroless runtime images (see references/security-hardening.md)
• NO → Use slim/alpine base images

3. Is image size critical?

• YES (<50MB) → Multi-stage + distroless + static linking
• NO (<500MB) → Multi-stage + slim base images

Core Concepts

Multi-Stage Builds

Separate build environment from runtime environment to minimize final image size.

Pattern:

# Stage 1: Build
FROM build-image AS builder
RUN compile application

# Stage 2: Runtime
FROM minimal-runtime-image
COPY --from=builder /app/binary /app/
CMD ["/app/binary"]

Benefits:

• 80-95% smaller images (excludes build tools)
• Improved security (no compilers in production)
• Faster deployments
• Better layer caching

Base Image Selection

Decision matrix:

| Language | Build Stage | Runtime Stage | Final Size |

|----------|-------------|---------------|------------|

| Go (static) | golang:1.22-alpine | gcr.io/distroless/static-debian12 | 10-30MB |

| Rust (static) | rust:1.75-alpine | scratch | 5-15MB |

| Python | python:3.12-slim | python:3.12-slim | 200-400MB |

| Node.js | node:20-alpine | node:20-alpine | 150-300MB |

| Java | maven:3.9-eclipse-temurin-21 | eclipse-temurin:21-jre-alpine | 200-350MB |

Distroless images (Google-maintained):

• gcr.io/distroless/static-debian12 → Static binaries (2MB)
• gcr.io/distroless/base-debian12 → Dynamic binaries with libc (20MB)
• gcr.io/distroless/python3-debian12 → Python runtime (60MB)
• gcr.io/distroless/nodejs20-debian12 → Node.js runtime (150MB)

See references/base-image-selection.md for complete comparison.

BuildKit Features

Enable BuildKit for advanced caching and security:

export DOCKER_BUILDKIT=1
docker build .
# OR
docker buildx build .

Key features:

• --mount=type=cache → Persistent package manager caches
• --mount=type=secret → Inject secrets without storing in layers
• --mount=type=ssh → SSH agent forwarding for private repos
• Parallel stage execution
• Improved layer caching

See references/buildkit-features.md for detailed patterns.

Layer Optimization

Order Dockerfile instructions from least to most frequently changing:

# 1. Base image (rarely changes)
FROM python:3.12-slim

# 2. System packages (rarely changes)
RUN apt-get update && apt-get install -y build-essential

# 3. Dependencies manifest (changes occasionally)
COPY requirements.txt .
RUN pip install -r requirements.txt

# 4. Application code (changes frequently)
COPY . .

# 5. Runtime configuration (rarely changes)
CMD ["python", "app.py"]

BuildKit cache mounts:

RUN --mount=type=cache,target=/root/.cache/pip \
    pip install -r requirements.txt

Cache persists across builds, eliminating redundant downloads.

Security Hardening

Essential security practices:

1. Non-root users

# Debian/Ubuntu
RUN useradd -m -u 1000 appuser && chown -R appuser:appuser /app
USER appuser

# Alpine
RUN adduser -D -u 1000 appuser && chown -R appuser:appuser /app
USER appuser

# Distroless (built-in)
USER nonroot:nonroot

2. Secret management

# ❌ NEVER: Secret in layer history
RUN git clone https://${GITHUB_TOKEN}@github.com/private/repo.git

# ✅ ALWAYS: BuildKit secret mount
RUN --mount=type=secret,id=github_token \
    TOKEN=$(cat /run/secrets/github_token) && \
    git clone https://${TOKEN}@github.com/private/repo.git

Build with:

docker buildx build --secret id=github_token,src=./token.txt .

3. Vulnerability scanning

# Trivy (recommended)
trivy image myimage:latest

# Docker Scout
docker scout cves myimage:latest

4. Health checks

HEALTHCHECK --interval=30s --timeout=3s --start-period=10s --retries=3 \
  CMD wget --no-verbose --tries=1 --spider http://localhost:8080/health || exit 1

See references/security-hardening.md for comprehensive hardening patterns.

.dockerignore Configuration

Create .dockerignore to exclude unnecessary files:

# Version control
.git
.gitignore

# CI/CD
.github
.gitlab-ci.yml

# IDE
.vscode
.idea

# Testing
tests/
coverage/
**/*_test.go
**/*.test.js

# Build artifacts
node_modules/
dist/
build/
target/
__pycache__/

# Environment
.env
.env.local
*.log

Reduces build context size and prevents leaking secrets.

Language-Specific Patterns

Python Quick Reference

Three approaches:

• pip (simple) → Single-stage, requirements.txt
• poetry (production) → Multi-stage, virtual environment
• uv (fastest) → 10-100x faster than pip

Example: Poetry multi-stage

FROM python:3.12-slim AS builder
RUN --mount=type=cache,target=/root/.cache/pip \
    pip install poetry==1.7.1

COPY pyproject.toml poetry.lock ./
RUN poetry export -f requirements.txt --output requirements.txt

RUN --mount=type=cache,target=/root/.cache/pip \
    python -m venv /opt/venv && \
    /opt/venv/bin/pip install -r requirements.txt

FROM python:3.12-slim
COPY --from=builder /opt/venv /opt/venv
ENV PATH="/opt/venv/bin:$PATH"
USER 1000:1000
CMD ["python", "-m", "uvicorn", "main:app", "--host", "0.0.0.0"]

See references/python-dockerfiles.md for complete patterns and examples/python-fastapi.Dockerfile.

Node.js Quick Reference

Key patterns:

• Use npm ci (not npm install) for reproducible builds
• Multi-stage: Build stage → Production dependencies only
• Built-in node user (UID 1000)
• Alpine variant smallest (~180MB vs 1GB)

Example: Express multi-stage

FROM node:20-alpine AS builder
WORKDIR /app
COPY package*.json ./
RUN --mount=type=cache,target=/root/.npm \
    npm ci
COPY . .
RUN npm run build
RUN npm prune --omit=dev

FROM node:20-alpine
WORKDIR /app
COPY --from=builder /app/node_modules ./node_modules
COPY --from=builder /app/dist ./dist
USER node
CMD ["node", "dist/index.js"]

See references/nodejs-dockerfiles.md for npm/pnpm/yarn patterns and examples/nodejs-express.Dockerfile.

Go Quick Reference

Smallest possible images:

• Static binary (CGO_ENABLED=0) + distroless = 10-30MB
• Strip symbols with -ldflags="-s -w"
• Cache both /go/pkg/mod and build cache

Example: Distroless static

FROM golang:1.22-alpine AS builder
WORKDIR /app
COPY go.mod go.sum ./
RUN --mount=type=cache,target=/go/pkg/mod \
    go mod download

COPY . .
RUN --mount=type=cache,target=/go/pkg/mod \
    --mount=type=cache,target=/root/.cache/go-build \
    CGO_ENABLED=0 GOOS=linux go build -ldflags="-s -w" -o main .

FROM gcr.io/distroless/static-debian12
COPY --from=builder /app/main /app/main
USER nonroot:nonroot
ENTRYPOINT ["/app/main"]

See references/go-dockerfiles.md and examples/go-microservice.Dockerfile.

Rust Quick Reference

Ultra-small static binaries:

• musl static linking → No libc dependencies
• scratch base image (0 bytes overhead)
• Final image: 5-15MB

Example: Scratch base

FROM rust:1.75-alpine AS builder
RUN apk add --no-cache musl-dev
WORKDIR /app

# Cache dependencies
COPY Cargo.toml Cargo.lock ./
RUN --mount=type=cache,target=/usr/local/cargo/registry \
    mkdir src && echo "fn main() {}" > src/main.rs && \
    cargo build --release --target x86_64-unknown-linux-musl && \
    rm -rf src

# Build application
COPY src ./src
RUN --mount=type=cache,target=/usr/local/cargo/registry \
    cargo build --release --target x86_64-unknown-linux-musl

FROM scratch
COPY --from=builder /app/target/x86_64-unknown-linux-musl/release/app /app
USER 1000:1000
ENTRYPOINT ["/app"]

See references/rust-dockerfiles.md and examples/rust-actix.Dockerfile.

Package Manager Cache Mounts

BuildKit cache mount locations:

| Language | Package Manager | Cache Mount Target |

|----------|----------------|-------------------|

| Python | pip | --mount=type=cache,target=/root/.cache/pip |

| Python | poetry | --mount=type=cache,target=/root/.cache/pypoetry |

| Python | uv | --mount=type=cache,target=/root/.cache/uv |

| Node.js | npm | --mount=type=cache,target=/root/.npm |

| Node.js | pnpm | --mount=type=cache,target=/root/.local/share/pnpm/store |

| Go | go mod | --mount=type=cache,target=/go/pkg/mod |

| Rust | cargo | --mount=type=cache,target=/usr/local/cargo/registry |

Persistent caches eliminate redundant package downloads across builds.

Validation and Testing

Validate Dockerfile quality:

# Lint Dockerfile
python scripts/validate_dockerfile.py Dockerfile

# Scan for vulnerabilities
trivy image myimage:latest

# Analyze image size
docker images myimage:latest
docker history myimage:latest

Compare optimization results:

# Before optimization
docker build -t myapp:before .

# After optimization
docker build -t myapp:after .

# Compare
bash scripts/analyze_image_size.sh myapp:before myapp:after

See scripts/validate_dockerfile.py for automated Dockerfile linting.

Integration with Related Skills

Upstream (provide input):

• testing-strategies → Test application before containerizing
• security-hardening → Application-level security before Docker layer

Downstream (consume Dockerfiles):

• building-ci-pipelines → Build and push Docker images in CI
• kubernetes-operations → Deploy containers to K8s clusters
• infrastructure-as-code → Deploy containers with Terraform/Pulumi

Parallel (related context):

• secret-management → Inject runtime secrets (K8s secrets, vaults)
• observability → Container logging and metrics collection

Common Patterns Quick Reference

1. Static binary (Go/Rust) → Smallest image

• Build: Language-specific builder image
• Runtime: gcr.io/distroless/static-debian12 or scratch
• Size: 5-30MB

2. Interpreted language (Python/Node.js) → Production-optimized

• Build: Install dependencies, build artifacts
• Runtime: Same base, production dependencies only
• Size: 150-400MB

3. JVM (Java) → Optimized runtime

• Build: Maven/Gradle with full JDK
• Runtime: JRE-only image (alpine variant)
• Size: 200-350MB

4. Security-critical → Maximum hardening

• Base: Distroless images
• User: Non-root (nonroot:nonroot)
• Secrets: BuildKit secret mounts
• Scan: Trivy/Docker Scout in CI

5. Development → Fast iteration

• Base: Full language image (not slim)
• Volumes: Mount source code
• Hot reload: Language-specific tools
• Not covered in this skill (see Docker Compose docs)

Anti-Patterns to Avoid

❌ Never:

• Use latest tags (unpredictable builds)
• Run as root in production
• Store secrets in ENV vars or layers
• Install unnecessary packages
• Combine unrelated RUN commands (breaks caching)
• Skip .dockerignore (bloated build context)

✅ Always:

• Pin exact image versions (python:3.12.1-slim, not python:3)
• Create and use non-root user
• Use BuildKit secret mounts for credentials
• Minimize layers and image size
• Order commands from least to most frequently changing
• Create .dockerignore file

Additional Resources

Base image registries:

• Google Distroless: gcr.io/distroless/*
• Docker Hub Official: python:*, node:*, golang:*
• Red Hat UBI: registry.access.redhat.com/ubi9/*

Vulnerability scanners:

• Trivy (recommended): trivy image myimage:latest
• Docker Scout: docker scout cves myimage:latest
• Grype: grype myimage:latest

Reference documentation:

• references/base-image-selection.md → Complete base image comparison
• references/buildkit-features.md → Advanced BuildKit patterns
• references/security-hardening.md → Comprehensive security guide
• Language-specific references in references/ directory
• Working examples in examples/ directory