Skip to content

SKILL.md

Latest commit

 

History

History
271 lines (219 loc) · 9.75 KB

SKILL.md

File metadata and controls

271 lines (219 loc) · 9.75 KB
name docker-ci-cd
description Use Docker in CI/CD pipelines with GitHub Actions, multi-platform builds, registry push, and layer caching strategies.
standards-version 1.7.0

Docker CI/CD

Trigger

Use this skill when:

  • Setting up Docker builds in GitHub Actions or GitLab CI
  • Building multi-platform images (amd64, arm64)
  • Pushing images to Docker Hub, GHCR, ECR, or other registries
  • Optimizing CI build times with layer caching
  • Defining a tagging strategy for releases
  • Setting up Docker-in-Docker for CI environments
  • Automating image scanning in the build pipeline

Required Inputs

  • CI platform (GitHub Actions, GitLab CI, Jenkins, CircleCI)
  • Target registry (Docker Hub, GHCR, ECR, GCR, ACR)
  • Target architectures (amd64, arm64, or both)
  • Tagging strategy (semver, git SHA, branch name, latest)
  • Whether vulnerability scanning should block deploys

Workflow

  1. Configure registry authentication - store credentials as CI secrets and log in during the pipeline.
  2. Set up buildx - use Docker Buildx for multi-platform builds and advanced caching.
  3. Define image tags - generate tags from git metadata (branch, tag, SHA) using docker/metadata-action or equivalent.
  4. Enable layer caching - use GitHub Actions cache, registry cache, or local cache to avoid rebuilding unchanged layers.
  5. Build and push - use docker/build-push-action or docker buildx build --push.
  6. Scan for vulnerabilities - run Trivy, Snyk, or Docker Scout after building and fail on critical CVEs.
  7. Test the image - pull the built image and run smoke tests before deploying.

Key References

Example Interaction

User: Set up a GitHub Actions workflow to build and push a Docker image to GHCR.

Agent: Here's a complete workflow with multi-platform builds, caching, and tagging:

# .github/workflows/docker.yml
name: Build and Push Docker Image

on:
  push:
    branches: [main]
    tags: ["v*"]
  pull_request:
    branches: [main]

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

jobs:
  build:
    runs-on: ubuntu-latest
    permissions:
      contents: read
      packages: write

    steps:
      - name: Checkout
        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: Log in to GHCR
        if: github.event_name != 'pull_request'
        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=
            type=ref,event=branch
            type=raw,value=latest,enable={{is_default_branch}}

      - name: Build and push
        uses: docker/build-push-action@v6
        with:
          context: .
          platforms: linux/amd64,linux/arm64
          push: ${{ github.event_name != 'pull_request' }}
          tags: ${{ steps.meta.outputs.tags }}
          labels: ${{ steps.meta.outputs.labels }}
          cache-from: type=gha
          cache-to: type=gha,mode=max

User: How do I add vulnerability scanning to that pipeline?

Agent: Add a scan step after the build:

      - name: Scan for vulnerabilities
        uses: aquasecurity/trivy-action@master
        with:
          image-ref: ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}:${{ steps.meta.outputs.version }}
          format: table
          exit-code: 1
          severity: CRITICAL,HIGH
          ignore-unfixed: true

Or use Docker Scout if you prefer the built-in option:

      - name: Docker Scout scan
        uses: docker/scout-action@v1
        with:
          command: cves
          image: ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}:${{ steps.meta.outputs.version }}
          only-severities: critical,high
          exit-code: true

User: What about GitLab CI?

Agent: Here's the GitLab equivalent using Docker-in-Docker:

# .gitlab-ci.yml
stages:
  - build

variables:
  DOCKER_TLS_CERTDIR: "/certs"
  IMAGE_TAG: $CI_REGISTRY_IMAGE:$CI_COMMIT_SHORT_SHA

build:
  stage: build
  image: docker:27
  services:
    - docker:27-dind
  before_script:
    - docker login -u $CI_REGISTRY_USER -p $CI_REGISTRY_PASSWORD $CI_REGISTRY
  script:
    - docker build
        --cache-from $CI_REGISTRY_IMAGE:latest
        --build-arg BUILDKIT_INLINE_CACHE=1
        -t $IMAGE_TAG
        -t $CI_REGISTRY_IMAGE:latest
        .
    - docker push $IMAGE_TAG
    - docker push $CI_REGISTRY_IMAGE:latest
  rules:
    - if: $CI_COMMIT_BRANCH == "main"

User: What caching strategies are available?

Agent: Three main options, each with tradeoffs:

# 1. GitHub Actions cache (gha) - fast, 10GB limit per repo
cache-from: type=gha
cache-to: type=gha,mode=max

# 2. Registry cache - unlimited size, stored alongside your image
cache-from: type=registry,ref=ghcr.io/myorg/myapp:buildcache
cache-to: type=registry,ref=ghcr.io/myorg/myapp:buildcache,mode=max

# 3. Local cache - useful for self-hosted runners
cache-from: type=local,src=/tmp/.buildx-cache
cache-to: type=local,dest=/tmp/.buildx-cache-new,mode=max

The mode=max flag caches all layers, not just the final image layers. This is critical for multi-stage builds where intermediate layers would otherwise be uncached.

User: How do I pass build args from CI secrets?

Agent:

      - name: Build with secrets
        uses: docker/build-push-action@v6
        with:
          context: .
          push: true
          tags: ${{ steps.meta.outputs.tags }}
          build-args: |
            APP_VERSION=${{ github.ref_name }}
            NODE_ENV=production
          secrets: |
            npmrc=${{ secrets.NPM_TOKEN }}

In the Dockerfile, consume the secret (not the build arg) for sensitive values:

RUN --mount=type=secret,id=npmrc,target=/root/.npmrc npm ci

Build args are fine for non-sensitive values like version strings. Never pass passwords or tokens as build args.

MCP Usage

Tool Purpose
docker_listImages Verify built images exist locally and check their sizes
docker_inspectImage Check labels, architecture, and layers of a built image
docker_searchHub Find official base images and their tags for CI pipelines
docker_systemInfo Verify buildx and multi-platform support is available
docker_diskUsage Monitor build cache size on self-hosted runners
docker_listContainers Check test containers spawned during CI smoke tests

Verifying a CI-built image locally:

1. docker_listImages - find the built image and check its size
2. docker_inspectImage - verify:
   - Architecture matches target platforms
   - Labels include version and commit SHA
   - Config.User is not root
   - No secrets in Config.Env
3. docker_listContainers - after smoke test, verify test containers ran

Checking build cache usage on runners:

Call docker_diskUsage to see how much space the build cache
consumes. If it's growing unbounded on self-hosted runners,
add a cache cleanup step to the pipeline.

Common Pitfalls

  • Not caching multi-stage intermediate layers - default cache mode only caches the final stage. Always use mode=max to cache all stages.
  • GitHub Actions cache eviction - the 10GB cache limit is shared across all workflows in a repo. Heavy caching can evict other workflow caches. Consider registry cache for large images.
  • QEMU builds are slow - building arm64 images on amd64 runners via QEMU emulation can be 5-10x slower. Use native arm64 runners for large builds if available.
  • Tagging with latest on every push - latest should only point to the most recent stable release. Tag PRs with the branch name or SHA, not latest.
  • Build args leaking secrets - ARG values are visible in docker history. Use --mount=type=secret for anything sensitive.
  • Docker-in-Docker socket security - mounting /var/run/docker.sock in CI gives the build container full control over the host's Docker daemon. Use Docker-in-Docker (dind) with TLS for better isolation.
  • Not pinning action versions - using @master for GitHub Actions can break workflows. Pin to a specific version tag or SHA.
  • Missing platform in the Dockerfile - FROM --platform=$BUILDPLATFORM is needed for cross-compilation stages. Without it, intermediate stages run under emulation.
  • Forgetting to log in before push - builds succeed but push fails with auth errors. Always add the login step before build-push.
  • Cache poisoning from PRs - GitHub Actions cache is shared. A malicious PR could push a poisoned cache. Use cache-from carefully and consider read-only cache for PR builds.
  • Ignoring build duration - CI builds over 10 minutes waste developer time and money. Profile with --progress=plain to find slow layers and optimize.

See Also