Docker Containers

Docker & Containers

Executive Summary

Production-grade container image building and management covering multi-stage builds, image optimization, security scanning, and registry operations. This skill provides deep expertise in creating minimal, secure, and efficient container images following industry best practices and OCI specifications.

Core Competencies

1. Production-Grade Multi-Stage Builds

Optimized Node.js Application

# Stage 1: Dependencies
FROM node:20-alpine AS deps
WORKDIR /app
COPY package*.json ./
RUN npm ci --only=production && npm cache clean --force

# Stage 2: Build
FROM node:20-alpine AS builder
WORKDIR /app
COPY package*.json ./
RUN npm ci
COPY . .
RUN npm run build

# Stage 3: Production
FROM gcr.io/distroless/nodejs20-debian12 AS production
WORKDIR /app
COPY --from=deps /app/node_modules ./node_modules
COPY --from=builder /app/dist ./dist
COPY --from=builder /app/package.json ./

ENV NODE_ENV=production
USER nonroot:nonroot
EXPOSE 3000
CMD ["dist/server.js"]

Optimized Go Application

# Stage 1: Build
FROM golang:1.22-alpine AS builder
RUN apk add --no-cache git ca-certificates tzdata
WORKDIR /app

# Cache dependencies
COPY go.mod go.sum ./
RUN go mod download && go mod verify

# Build
COPY . .
RUN CGO_ENABLED=0 GOOS=linux GOARCH=amd64 go build \
    -ldflags="-w -s -X main.version=${VERSION}" \
    -o /app/server ./cmd/server

# Stage 2: Production (scratch)
FROM scratch AS production
COPY --from=builder /etc/ssl/certs/ca-certificates.crt /etc/ssl/certs/
COPY --from=builder /usr/share/zoneinfo /usr/share/zoneinfo
COPY --from=builder /app/server /server

USER 65534:65534
EXPOSE 8080
ENTRYPOINT ["/server"]

Optimized Python Application

# Stage 1: Dependencies
FROM python:3.12-slim AS builder
WORKDIR /app

RUN pip install --no-cache-dir poetry && \
    poetry config virtualenvs.in-project true

COPY pyproject.toml poetry.lock ./
RUN poetry install --only main --no-interaction --no-ansi

# Stage 2: Production
FROM python:3.12-slim AS production
WORKDIR /app

# Create non-root user
RUN groupadd -r appgroup && useradd -r -g appgroup appuser

# Copy virtual environment
COPY --from=builder /app/.venv /app/.venv
COPY . .

ENV PATH="/app/.venv/bin:$PATH"
ENV PYTHONDONTWRITEBYTECODE=1
ENV PYTHONUNBUFFERED=1

USER appuser:appgroup
EXPOSE 8000
CMD ["gunicorn", "-w", "4", "-b", "0.0.0.0:8000", "app:app"]

2. Image Optimization Strategies

Layer Optimization Checklist

┌─────────────────────────────────────────────────────────────┐
│              IMAGE OPTIMIZATION CHECKLIST                    │
├─────────────────────────────────────────────────────────────┤
│ ✓ Use minimal base images (alpine, distroless, scratch)    │
│ ✓ Multi-stage builds to exclude build tools                │
│ ✓ Combine RUN commands to reduce layers                    │
│ ✓ Order layers by change frequency (least → most)          │
│ ✓ Use .dockerignore to exclude unnecessary files           │
│ ✓ Clean package manager cache in same RUN command          │
│ ✓ Use specific version tags, avoid :latest                 │
│ ✓ Set appropriate user permissions                         │
│ ✓ Use COPY instead of ADD when possible                    │
│ ✓ Leverage build cache effectively                         │
└─────────────────────────────────────────────────────────────┘

Optimized .dockerignore

# Git
.git
.gitignore

# Dependencies
node_modules
.venv
vendor

# Build artifacts
dist
build
*.egg-info

# IDE and editors
.idea
.vscode
*.swp
*.swo

# Testing
coverage
.pytest_cache
__pycache__

# Documentation
docs
*.md
!README.md

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

# Docker
Dockerfile*
docker-compose*
.docker

# Secrets and config
.env*
*.pem
*.key
secrets/

Base Image Comparison

┌──────────────────┬────────────┬─────────────────────────────────┐
│ Base Image       │ Size       │ Use Case                        │
├──────────────────┼────────────┼─────────────────────────────────┤
│ ubuntu:22.04     │ ~77MB      │ Debugging, full tooling         │
│ debian:slim      │ ~52MB      │ Compatibility, some tools       │
│ alpine:3.19      │ ~7MB       │ Small size, musl libc           │
│ distroless       │ ~2-20MB    │ Minimal, no shell               │
│ scratch          │ 0MB        │ Static binaries only            │
│ chainguard       │ ~2-10MB    │ Security-focused, maintained    │
└──────────────────┴────────────┴─────────────────────────────────┘

3. Container Security

Security-Hardened Dockerfile

FROM node:20-alpine AS builder
# ... build steps ...

FROM gcr.io/distroless/nodejs20-debian12

# Use digest for immutability
# FROM gcr.io/distroless/nodejs20-debian12@sha256:abc123...

WORKDIR /app

# Copy only necessary files
COPY --from=builder --chown=nonroot:nonroot /app/dist ./dist
COPY --from=builder --chown=nonroot:nonroot /app/node_modules ./node_modules
COPY --from=builder --chown=nonroot:nonroot /app/package.json ./

# Security labels
LABEL org.opencontainers.image.source="https://github.com/org/repo"
LABEL org.opencontainers.image.licenses="MIT"
LABEL org.opencontainers.image.vendor="MyCompany"

# Run as non-root
USER nonroot:nonroot

# Read-only filesystem compatible
VOLUME ["/tmp"]

ENV NODE_ENV=production
EXPOSE 3000

HEALTHCHECK --interval=30s --timeout=3s --start-period=5s --retries=3 \
  CMD ["/nodejs/bin/node", "-e", "require('http').get('http://localhost:3000/health')"]

ENTRYPOINT ["/nodejs/bin/node"]
CMD ["dist/server.js"]

Image Scanning with Trivy

# Scan image for vulnerabilities
trivy image --severity HIGH,CRITICAL myapp:latest

# Scan with SARIF output for GitHub
trivy image --format sarif --output results.sarif myapp:latest

# Scan and fail on critical
trivy image --exit-code 1 --severity CRITICAL myapp:latest

# Generate SBOM
trivy image --format cyclonedx --output sbom.json myapp:latest

Image Signing with Cosign

# Generate key pair
cosign generate-key-pair

# Sign image
cosign sign --key cosign.key myregistry.io/myapp:v1.0.0

# Verify signature
cosign verify --key cosign.pub myregistry.io/myapp:v1.0.0

# Keyless signing (GitHub Actions)
cosign sign --yes \
  --oidc-issuer=https://token.actions.githubusercontent.com \
  myregistry.io/myapp:${{ github.sha }}

4. Registry Management

Multi-Registry Push Configuration

# Build with BuildKit
DOCKER_BUILDKIT=1 docker build \
  --platform linux/amd64,linux/arm64 \
  --push \
  --cache-from type=registry,ref=myregistry.io/myapp:cache \
  --cache-to type=registry,ref=myregistry.io/myapp:cache,mode=max \
  -t myregistry.io/myapp:v1.0.0 \
  -t myregistry.io/myapp:latest \
  .

# Push to multiple registries
for registry in docker.io gcr.io ghcr.io; do
  docker tag myapp:v1.0.0 $registry/myorg/myapp:v1.0.0
  docker push $registry/myorg/myapp:v1.0.0
done

Registry Authentication in Kubernetes

# ImagePullSecret for private registry
apiVersion: v1
kind: Secret
metadata:
  name: regcred
  namespace: production
type: kubernetes.io/dockerconfigjson
data:
  .dockerconfigjson: |
    eyJhdXRocyI6eyJteXJlZ2lzdHJ5LmF6dXJlY3IuaW8iOnsiYXV0aCI6IlkyeH...
---
# Usage in Deployment
apiVersion: apps/v1
kind: Deployment
metadata:
  name: myapp
spec:
  template:
    spec:
      imagePullSecrets:
      - name: regcred
      containers:
      - name: app
        image: myregistry.azurecr.io/myapp:v1.0.0@sha256:abc123...

5. Build Optimization

BuildKit Caching Strategies

# syntax=docker/dockerfile:1.6

FROM node:20-alpine AS builder

# Mount cache for npm
RUN --mount=type=cache,target=/root/.npm \
    npm ci

# Mount secrets without embedding in image
RUN --mount=type=secret,id=npmrc,target=/root/.npmrc \
    npm install @private/package

# Mount ssh for private repos
RUN --mount=type=ssh \
    git clone git@github.com:org/private-repo.git

GitHub Actions CI/CD

name: Build and Push

on:
  push:
    branches: [main]
    tags: ['v*']

env:
  REGISTRY: ghcr.io
  IMAGE_NAME: ${{ github.repository }}

jobs:
  build:
    runs-on: ubuntu-latest
    permissions:
      contents: read
      packages: write
      id-token: write  # For keyless signing
      security-events: write  # For SARIF upload

    steps:
    - uses: actions/checkout@v4

    - name: Set up QEMU
      uses: docker/setup-qemu-action@v3

    - name: Set up Docker Buildx
      uses: docker/setup-buildx-action@v3

    - name: Login to Container Registry
      uses: docker/login-action@v3
      with:
        registry: ${{ env.REGISTRY }}
        username: ${{ github.actor }}
        password: ${{ secrets.GITHUB_TOKEN }}

    - name: Extract metadata
      id: meta
      uses: docker/metadata-action@v5
      with:
        images: ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}
        tags: |
          type=semver,pattern={{version}}
          type=semver,pattern={{major}}.{{minor}}
          type=sha,prefix=

    - name: Build and push
      uses: docker/build-push-action@v5
      with:
        context: .
        platforms: linux/amd64,linux/arm64
        push: true
        tags: ${{ steps.meta.outputs.tags }}
        labels: ${{ steps.meta.outputs.labels }}
        cache-from: type=gha
        cache-to: type=gha,mode=max
        provenance: true
        sbom: true

    - name: Run Trivy vulnerability scanner
      uses: aquasecurity/trivy-action@master
      with:
        image-ref: '${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}:${{ github.sha }}'
        format: 'sarif'
        output: 'trivy-results.sarif'
        severity: 'CRITICAL,HIGH'

    - name: Upload Trivy scan results
      uses: github/codeql-action/upload-sarif@v2
      with:
        sarif_file: 'trivy-results.sarif'

    - name: Install Cosign
      uses: sigstore/cosign-installer@v3

    - name: Sign image
      run: |
        cosign sign --yes ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}@${{ steps.build.outputs.digest }}

Integration Patterns

Used by skill: cluster-admin

• Node container runtime configuration
• Registry credentials management
• Image pull policies

Coordinates with skill: security

• Image vulnerability scanning
• Signature verification
• Supply chain security

Works with skill: gitops

• Image promotion workflows
• Automated deployments
• Version management

Troubleshooting Guide

Decision Tree: Build Issues

Docker Build Problem?
│
├── Build fails at COPY/ADD
│   ├── Check: file exists in build context
│   ├── Check: .dockerignore not excluding file
│   └── Verify: correct relative paths
│
├── Build slow
│   ├── Check: layer ordering (frequent changes last)
│   ├── Enable: BuildKit (DOCKER_BUILDKIT=1)
│   ├── Use: cache mounts for package managers
│   └── Verify: .dockerignore excludes large files
│
├── Image too large
│   ├── Use: multi-stage builds
│   ├── Switch: to smaller base image
│   ├── Clean: caches in same RUN command
│   └── Analyze: with dive tool
│
└── Security vulnerabilities
    ├── Update: base image to latest patch
    ├── Minimize: installed packages
    ├── Use: distroless or scratch
    └── Scan: regularly with Trivy

Debug Commands Cheatsheet

# Inspect image layers and size
docker history myapp:latest --no-trunc
dive myapp:latest  # Interactive layer analysis

# Debug build
DOCKER_BUILDKIT=1 docker build --progress=plain -t myapp .
docker build --no-cache -t myapp .

# Inspect image metadata
docker inspect myapp:latest
docker inspect --format='{{.Config.Labels}}' myapp:latest
skopeo inspect docker://myregistry.io/myapp:latest

# Test image locally
docker run --rm -it --entrypoint=/bin/sh myapp:latest
docker run --rm myapp:latest cat /etc/os-release

# Check for secrets in image
docker history --no-trunc myapp:latest | grep -i secret
trivy image --scanners secret myapp:latest

# Analyze vulnerabilities
trivy image --severity CRITICAL myapp:latest
grype myapp:latest

# Export filesystem
docker export $(docker create myapp:latest) | tar -tvf - | head -100

Common Challenges & Solutions

Success Criteria

Resources

• Docker Documentation
• Best Practices for Writing Dockerfiles
• Distroless Images
• Trivy Scanner
• Cosign Signing