Harness Containerization

Dockerfile review, Kubernetes manifest validation, and container optimization. Smaller images, safer containers, correct orchestration.

When to Use

• When reviewing Dockerfiles for image size, security, and layer efficiency
• When auditing Kubernetes manifests, Helm charts, or docker-compose files
• On PRs that modify container configuration files
• NOT for CI/CD pipeline design (use harness-deployment)
• NOT for infrastructure provisioning (use harness-infrastructure-as-code)
• NOT for application-level security review (use harness-security-review)

Process

Phase 1: SCAN -- Discover Container Configuration

1. Locate container files. Search the project for container-related configuration:

   • Dockerfile

     ,

     Dockerfile.*

     (multi-target builds)

   • docker-compose.yml

     ,

     docker-compose.*.yml

     (override files)

   • .dockerignore

   • k8s/

     ,

     kubernetes/

     ,

     manifests/

     directories

   • helm/

     ,

     charts/

     directories

   • skaffold.yaml

     ,

     tilt.json

     (dev tooling)

2. Identify base images. Parse each Dockerfile for FROM directives:

   • Record base image name, tag, and digest (if pinned)
   • Flag images using

     latest

     tag
   • Flag images from untrusted registries
   • Note multi-stage build structure (builder vs. runtime stages)

3. Inventory Kubernetes resources. Parse manifest files and record:

   • Resource types (Deployment, Service, ConfigMap, Secret, Ingress, HPA)
   • Namespaces used
   • Image references in pod specs
   • Resource requests and limits
   • Volume mounts and persistent volume claims

4. Detect Helm usage. If Helm charts exist:

   • Parse

     Chart.yaml

     for version and dependencies
   • Parse

     values.yaml

     for configurable parameters
   • Identify template files and their output resource types

5. Present scan summary:

   Container Scan:
     Dockerfiles: 2 (app, worker)
     Compose files: 1 (docker-compose.yml + docker-compose.dev.yml)
     K8s manifests: 8 resources across 2 namespaces
     Helm charts: 1 (app chart with 3 subcharts)
     Base images: node:20-alpine, python:3.12-slim
   

Phase 2: ANALYZE -- Evaluate Best Practices

1. Analyze Dockerfile layer efficiency. Check each Dockerfile for:

   • COPY/ADD placement relative to dependency installation (cache busting)
   • Multi-stage builds separating build dependencies from runtime
   • Layer count optimization (combining related RUN commands)
   • Unnecessary files copied into the image (node_modules, .git, tests)

   • .dockerignore

     completeness

2. Check container security posture. Evaluate:

   • Running as non-root user (USER directive present)
   • No secrets in build args or environment variables
   • Base image currency (is the tag reasonably current)
   • HEALTHCHECK directive present
   • Read-only filesystem where possible
   • No privileged mode in compose or K8s specs
   • Security contexts in Kubernetes pod specs (runAsNonRoot, readOnlyRootFilesystem)

3. Evaluate Kubernetes resource definitions. For each Deployment/StatefulSet:

   • Resource requests and limits are set (CPU and memory)
   • Liveness and readiness probes are configured
   • Pod disruption budgets exist for production workloads
   • Horizontal pod autoscaler is configured where appropriate
   • Image pull policy is set (Always for mutable tags, IfNotPresent for digests)

4. Analyze docker-compose configuration. Check for:

   • Service dependency ordering (depends_on with health checks)
   • Volume mount correctness (host paths vs. named volumes)
   • Network isolation between services
   • Environment variable management (env_file vs. inline)
   • Port mapping conflicts

5. Check image tag strategy. Verify:

   • Production images use immutable tags (semver or digest)
   • Development images use descriptive tags (branch name, commit SHA)
   • No

     latest

     tag in production manifests
   • Registry URL is consistent across all references

Phase 3: OPTIMIZE -- Recommend Improvements

1. Recommend image size reduction. For each Dockerfile:

   • Switch to minimal base images (alpine, distroless, scratch)
   • Remove build-only dependencies in multi-stage builds
   • Use

     .dockerignore

     to exclude test files, docs, and dev configs
   • Estimate size savings for each recommendation

2. Recommend build performance improvements.

   • Reorder COPY directives to maximize layer cache hits

   • Use BuildKit features (cache mounts for package managers)

   • Split slow-changing layers (OS packages) from fast-changing layers (app code)

   • Example for Node.js:

     # Good: dependency layer cached separately
     COPY package.json package-lock.json ./
     RUN npm ci --production
     COPY src/ ./src/
     

3. Recommend Kubernetes improvements.

   • Add missing resource limits with reasonable defaults
   • Configure probes with appropriate initial delays and periods
   • Add pod anti-affinity for high-availability workloads
   • Recommend namespace isolation for multi-tenant clusters
   • Add network policies to restrict pod-to-pod communication

4. Recommend security hardening.

   • Add non-root USER directive with specific UID
   • Add security context to Kubernetes pods
   • Pin base images to digest for supply chain security
   • Remove unnecessary capabilities (drop ALL, add only what is needed)

5. Generate optimization summary with estimated impact:

   Optimization Summary:
     Image size: 850MB -> ~180MB (switch to alpine + multi-stage)
     Build time: ~4m -> ~2m (layer reordering + cache mounts)
     Security: 3 findings (non-root, capabilities, image pinning)
     K8s: 5 resources missing resource limits
   

Phase 4: VALIDATE -- Verify Configuration Correctness

1. Validate Dockerfile syntax. Run

   docker build --check

   or parse for common errors:
   • Invalid instruction ordering (e.g., CMD before COPY)
   • Missing required arguments
   • Deprecated instructions (MAINTAINER)
   • Shell form vs. exec form for CMD/ENTRYPOINT

2. Validate Kubernetes manifests. Check for:

   • Valid YAML structure
   • Required fields present (apiVersion, kind, metadata, spec)
   • Label selectors match between Deployment and Service
   • Port numbers are consistent across Service and container specs
   • ConfigMap and Secret references resolve to existing resources

3. Validate Helm charts. If Helm is used:

   • helm lint

     passes
   • Template rendering with default values produces valid manifests
   • Values schema matches actual usage in templates
   • Dependencies are declared and version-locked

4. Validate docker-compose. Check for:

   • Valid YAML and compose file version
   • All referenced images exist or have build contexts
   • Port mappings do not conflict
   • Named volumes are declared in the top-level volumes section
   • Networks are declared before use

5. Generate validation report:

   Container Validation: [PASS/WARN/FAIL]
   
   Dockerfiles: PASS (2/2 valid)
   K8s manifests: WARN (label mismatch in worker-service.yaml)
   Helm chart: PASS (lint clean)
   Compose: PASS (valid structure)
   
   Issues:
     1. k8s/worker-service.yaml: selector "app: worker" does not match
        deployment label "app: worker-v2" -- requests will not route
   

Harness Integration

• harness skill run harness-containerization

  -- Primary invocation for container review.

• harness validate

  -- Run after configuration changes to verify project health.

• harness check-deps

  -- Verify container tooling dependencies are available.

• emit_interaction

  -- Present optimization recommendations and gather decisions.

Success Criteria

• All container configuration files in the project are discovered and cataloged
• Dockerfiles are analyzed for layer efficiency, security, and size
• Kubernetes manifests are validated for correctness and best practices
• Resource requests and limits are verified for all production workloads
• Image tag strategy is evaluated (no

  latest

  in production)
• Optimization recommendations include estimated impact

Examples

Example: Node.js Monorepo with Docker and Kubernetes

Phase 1: SCAN
  Found: Dockerfile (app), Dockerfile.worker, docker-compose.dev.yml
  K8s: 12 manifests in k8s/ (2 Deployments, 2 Services, 2 ConfigMaps,
       2 HPA, 2 Ingress, 2 PDB)
  Base images: node:20 (not alpine), node:20 (worker)

Phase 2: ANALYZE
  Dockerfile issues:
    - node:20 full image (940MB) -- use node:20-alpine (180MB)
    - No .dockerignore -- node_modules and .git copied into image
    - No USER directive -- running as root
    - No HEALTHCHECK
  K8s issues:
    - worker deployment missing memory limits
    - No network policies defined
    - Liveness probe on /healthz but no readiness probe

Phase 3: OPTIMIZE
  1. Switch to node:20-alpine -- saves ~760MB per image
  2. Add .dockerignore with node_modules, .git, tests, docs
  3. Add multi-stage build: builder stage for npm ci, runtime for app
  4. Add USER node (UID 1000) after COPY
  5. Add readiness probe on /ready endpoint
  6. Add memory limit of 512Mi to worker deployment

Phase 4: VALIDATE
  Dockerfiles: WARN (2 security findings, 1 size finding)
  K8s manifests: WARN (missing limits, missing readiness probe)
  Compose: PASS
  Result: WARN -- 6 actionable improvements identified

Example: Python FastAPI with Helm and Distroless

Phase 1: SCAN
  Found: Dockerfile (multi-stage with distroless runtime)
  Helm chart: charts/api/ with values.yaml
  Base images: python:3.12-slim (builder), gcr.io/distroless/python3 (runtime)

Phase 2: ANALYZE
  Dockerfile: Well-structured multi-stage build
    - Builder installs dependencies, runtime copies only venv
    - Distroless base (no shell, minimal attack surface)
    - Non-root user configured
  Helm:
    - Resource limits set in values.yaml
    - Probes configured with appropriate timeouts
    - HPA configured for 2-10 replicas

Phase 3: OPTIMIZE
  Minor recommendations only:
    - Pin distroless image to digest for reproducibility
    - Add --mount=type=cache for pip downloads in builder stage
    - Add pod anti-affinity to spread replicas across nodes

Phase 4: VALIDATE
  Dockerfile: PASS
  Helm lint: PASS
  Template render: PASS (all values resolve)
  Result: PASS -- well-configured container setup

Rationalizations to Reject

latest

latest

--cap-add NET_BIND_SERVICE

Gates

• No

  latest

  tag in production manifests. Production Kubernetes manifests or compose files using

  latest

  image tags are blocking findings. Immutable tags or digests are required.
• No containers running as root in production. Missing USER directive in Dockerfiles or missing security context in K8s pods targeting production are blocking findings.
• No missing resource limits in production. Kubernetes Deployments without CPU and memory limits are blocking warnings for production namespaces.
• No invalid manifest references. Label selector mismatches between Services and Deployments, or ConfigMap/Secret references to nonexistent resources, are blocking errors.

Escalation

• When base images have known CVEs: Flag the specific CVEs and recommend upgrading to a patched version. If no patched version exists, recommend an alternative base image and document the migration path.
• When Kubernetes manifest complexity exceeds review scope: For clusters with 50+ resources, recommend focusing on changed resources only (

  --changed-only

  flag) and scheduling a full audit separately.
• When Helm chart dependencies are outdated: Report the version gap and recommend updating. If the update includes breaking changes, flag it as a decision point and present the changelog.
• When docker-compose is used for production: Flag this as an architectural concern. Docker Compose is appropriate for development but production workloads should use an orchestrator (Kubernetes, ECS, Cloud Run). Present migration options.