Container Security Guide

name: container-security-guide description: Docker and Kubernetes security hardening guide covering images, builds, runtime, and orchestration

Container Security Guide

Overview

Containers introduce a unique attack surface spanning the image build pipeline, container runtime, orchestration layer, and host OS. Security must be applied at every stage of the container lifecycle:

This guide covers Docker image and build security, Kubernetes-specific hardening, and runtime protection. It is tailored for teams running containerized workloads on GKE but applies broadly to any Kubernetes distribution.

When to Use This Skill

• You are writing or reviewing Dockerfiles for production services
• You are configuring Kubernetes security settings (PodSecurityStandards, NetworkPolicies, RBAC)
• You need to harden a container runtime environment against known attack vectors
• You are building a CI/CD pipeline and need image scanning and signing gates
• You are investigating a container security incident or preparing for an audit
• You want to implement defense-in-depth for containerized microservices

How It Works

Step 1: Scan Container Images

Scan images for known vulnerabilities before they reach production:

# Trivy: comprehensive vulnerability scanner
trivy image --severity HIGH,CRITICAL faos-api:latest

# Grype: fast alternative scanner
grype faos-api:latest --only-fixed --fail-on high

# Snyk: SaaS scanner with fix suggestions
snyk container test faos-api:latest --severity-threshold=high

# GCP Artifact Registry: automatic scanning
gcloud artifacts docker images list-vulnerabilities \
  REGION-docker.pkg.dev/PROJECT/REPO/my-app:latest \
  --format=json | jq '.[] | select(.vulnerability.effectiveSeverity == "CRITICAL")'

Integrate scanning into CI/CD pipelines with a gate:

# GitHub Actions: scan and fail on critical vulnerabilities
- name: Scan container image
  uses: aquasecurity/trivy-action@master
  with:
    image-ref: ${{ env.IMAGE }}
    format: 'sarif'
    output: 'trivy-results.sarif'
    severity: 'CRITICAL,HIGH'
    exit-code: '1'  # Fail the build on findings

Step 2: Harden Dockerfiles

Build minimal, secure container images following these principles:

• Use minimal base images (distroless, Alpine, or scratch)
• Run as non-root user
• Use multi-stage builds to exclude build tools from production images
• Never embed secrets, credentials, or private keys in images
• Pin base image versions with digest for reproducibility
• Set the filesystem to read-only where possible
• Include health checks for orchestrator integration

Step 3: Apply PodSecurityStandards

Kubernetes PodSecurityStandards (PSS) define three progressive security profiles:

Apply PSS enforcement at the namespace level:

# Enforce restricted profile on production namespace
kubectl label namespace faos-api \
  pod-security.kubernetes.io/enforce=restricted \
  pod-security.kubernetes.io/enforce-version=latest \
  pod-security.kubernetes.io/warn=restricted \
  pod-security.kubernetes.io/audit=restricted

Step 4: Configure NetworkPolicies

By default, all pods in a Kubernetes cluster can communicate with all other pods. NetworkPolicies implement microsegmentation:

• Start with a default-deny policy for all namespaces
• Explicitly allow only required communication paths
• Use label selectors for fine-grained pod-to-pod rules
• Allow DNS resolution (kube-dns) in every policy

Step 5: Enable Runtime Monitoring

Runtime security tools detect anomalous container behavior:

# Falco: runtime threat detection
helm repo add falcosecurity https://falcosecurity.github.io/charts
helm install falco falcosecurity/falco \
  --namespace falco-system \
  --create-namespace \
  --set falcosidekick.enabled=true \
  --set falcosidekick.config.slack.webhookurl=$SLACK_WEBHOOK

# Falco detects:
# - Shell spawned in container
# - Sensitive file access (/etc/shadow, /etc/passwd)
# - Unexpected network connections
# - Privilege escalation attempts
# - Cryptocurrency miner signatures

Runtime protection layers:

Examples

Example 1: Secure Dockerfile for Python FastAPI App

# ============================================
# Stage 1: Build dependencies
# ============================================
FROM python:3.12-slim AS builder

# Install build dependencies
RUN apt-get update && apt-get install -y --no-install-recommends \
    gcc \
    libpq-dev \
    && rm -rf /var/lib/apt/lists/*

# Create virtual environment
RUN python -m venv /opt/venv
ENV PATH="/opt/venv/bin:$PATH"

# Install Python dependencies
COPY requirements.txt .
RUN pip install --no-cache-dir --upgrade pip && \
    pip install --no-cache-dir -r requirements.txt

# ============================================
# Stage 2: Production image
# ============================================
FROM python:3.12-slim AS production

# Security: Create non-root user
RUN groupadd -r appuser && useradd -r -g appuser -d /app -s /sbin/nologin appuser

# Install only runtime dependencies (no gcc/build tools)
RUN apt-get update && apt-get install -y --no-install-recommends \
    libpq5 \
    curl \
    && rm -rf /var/lib/apt/lists/*

# Copy virtual environment from builder
COPY --from=builder /opt/venv /opt/venv
ENV PATH="/opt/venv/bin:$PATH"

# Copy application code
WORKDIR /app
COPY --chown=appuser:appuser ./src ./src

# Security: No secrets in image (use env vars or mounted secrets)
# Security: Read-only filesystem compatible
# Security: Drop all capabilities in K8s securityContext

# Health check for Kubernetes probes
HEALTHCHECK --interval=30s --timeout=5s --start-period=10s --retries=3 \
    CMD curl -f http://localhost:8000/health || exit 1

# Security: Run as non-root user
USER appuser

# Security: Use exec form (no shell injection risk)
ENTRYPOINT ["uvicorn", "src.main:app"]
CMD ["--host", "0.0.0.0", "--port", "8000", "--workers", "4"]

Security features in this Dockerfile:

Example 2: Kubernetes NetworkPolicy for Namespace Isolation

# 1. Default deny all ingress and egress traffic
apiVersion: networking.k8s.io/v1
kind: NetworkPolicy
metadata:
  name: default-deny-all
  namespace: faos-api
spec:
  podSelector: {}  # Applies to all pods in namespace
  policyTypes:
    - Ingress
    - Egress

---
# 2. Allow DNS resolution (required for service discovery)
apiVersion: networking.k8s.io/v1
kind: NetworkPolicy
metadata:
  name: allow-dns
  namespace: faos-api
spec:
  podSelector: {}
  policyTypes:
    - Egress
  egress:
    - to:
        - namespaceSelector:
            matchLabels:
              kubernetes.io/metadata.name: kube-system
      ports:
        - protocol: UDP
          port: 53
        - protocol: TCP
          port: 53

---
# 3. Allow ingress from gateway to API pods only
apiVersion: networking.k8s.io/v1
kind: NetworkPolicy
metadata:
  name: allow-gateway-to-api
  namespace: faos-api
spec:
  podSelector:
    matchLabels:
      app: faos-api
  policyTypes:
    - Ingress
  ingress:
    - from:
        - namespaceSelector:
            matchLabels:
              kubernetes.io/metadata.name: faos-gateway
          podSelector:
            matchLabels:
              app: gateway
      ports:
        - protocol: TCP
          port: 8000

---
# 4. Allow API pods to reach database
apiVersion: networking.k8s.io/v1
kind: NetworkPolicy
metadata:
  name: allow-api-to-database
  namespace: faos-api
spec:
  podSelector:
    matchLabels:
      app: faos-api
  policyTypes:
    - Egress
  egress:
    - to:
        - ipBlock:
            cidr: 10.0.0.0/8  # Cloud SQL private IP range
      ports:
        - protocol: TCP
          port: 5432  # PostgreSQL

---
# 5. Allow API pods to reach Redis
apiVersion: networking.k8s.io/v1
kind: NetworkPolicy
metadata:
  name: allow-api-to-redis
  namespace: faos-api
spec:
  podSelector:
    matchLabels:
      app: faos-api
  policyTypes:
    - Egress
  egress:
    - to:
        - namespaceSelector:
            matchLabels:
              kubernetes.io/metadata.name: faos-cache
          podSelector:
            matchLabels:
              app: redis
      ports:
        - protocol: TCP
          port: 6379

Example 3: PodSecurityStandard (Restricted) Configuration

# Pod specification compliant with PSS restricted profile
apiVersion: apps/v1
kind: Deployment
metadata:
  name: faos-api
  namespace: faos-api
spec:
  replicas: 3
  selector:
    matchLabels:
      app: faos-api
  template:
    metadata:
      labels:
        app: faos-api
    spec:
      # Security: Use dedicated service account (not default)
      serviceAccountName: my-app-sa
      automountServiceAccountToken: false  # Disable unless needed

      # Security: Prevent privilege escalation via hostPID/hostNetwork
      hostPID: false
      hostNetwork: false
      hostIPC: false

      # Security: Set filesystem group for shared volumes
      securityContext:
        runAsNonRoot: true
        runAsUser: 65534      # nobody user
        runAsGroup: 65534
        fsGroup: 65534
        seccompProfile:
          type: RuntimeDefault

      containers:
        - name: faos-api
          image: REGION-docker.pkg.dev/PROJECT/REPO/my-app@sha256:abc123...
          ports:
            - containerPort: 8000
              protocol: TCP

          # Security: Container-level security context
          securityContext:
            allowPrivilegeEscalation: false
            readOnlyRootFilesystem: true
            runAsNonRoot: true
            runAsUser: 65534
            capabilities:
              drop:
                - ALL
            seccompProfile:
              type: RuntimeDefault

          # Resource limits (prevent resource abuse)
          resources:
            requests:
              cpu: 250m
              memory: 256Mi
            limits:
              cpu: 1000m
              memory: 512Mi

          # Health probes
          livenessProbe:
            httpGet:
              path: /health
              port: 8000
            initialDelaySeconds: 10
            periodSeconds: 30
          readinessProbe:
            httpGet:
              path: /ready
              port: 8000
            initialDelaySeconds: 5
            periodSeconds: 10

          # Mount secrets from external secret manager (not K8s Secrets)
          env:
            - name: DATABASE_URL
              valueFrom:
                secretKeyRef:
                  name: faos-api-secrets
                  key: database-url

          # Writable directories for temp files (read-only root filesystem)
          volumeMounts:
            - name: tmp
              mountPath: /tmp
            - name: cache
              mountPath: /app/.cache

      volumes:
        - name: tmp
          emptyDir:
            sizeLimit: 100Mi
        - name: cache
          emptyDir:
            sizeLimit: 50Mi

Best Practices

Do This

• Use distroless or slim base images to minimize the attack surface
• Scan images in CI/CD and block deployments with critical or high vulnerabilities
• Sign images with cosign or Notary and verify signatures with Binary Authorization
• Run all containers as non-root with runAsNonRoot: true
• Drop all Linux capabilities with capabilities.drop: [ALL]
• Set readOnlyRootFilesystem: true and mount writable directories as emptyDir volumes
• Apply PodSecurityStandards at restricted level for production namespaces
• Implement default-deny NetworkPolicies in every namespace
• Use external secret managers (GCP Secret Manager, HashiCorp Vault) instead of K8s Secrets
• Pin image references by digest (@sha256:...) rather than mutable tags
• Enable Seccomp profiles (at minimum RuntimeDefault) for all containers
• Implement pod disruption budgets alongside security to maintain availability

Don't Do This

• Do not use latest tag for production images -- it is mutable and unpredictable
• Do not run containers as root or with privileged: true
• Do not embed secrets, API keys, or credentials in Docker images
• Do not expose the Docker socket (/var/run/docker.sock) to containers
• Do not use hostPID, hostNetwork, or hostIPC unless absolutely required
• Do not skip NetworkPolicies -- default Kubernetes networking allows unrestricted pod-to-pod traffic
• Do not grant cluster-admin ClusterRole to application service accounts
• Do not auto-mount service account tokens (automountServiceAccountToken: false by default)
• Do not install shells (bash, sh) in production images if they are not needed
• Do not use package managers (apt, apk) at runtime -- install everything at build time

Security Checklist

Image Security

• Base images are minimal (distroless, Alpine, or slim variants)
• Images are scanned for vulnerabilities in CI/CD with a blocking gate
• No secrets, credentials, or private keys are embedded in images
• Multi-stage builds exclude build tools from production images
• Images are signed and verified before deployment (cosign / Binary Authorization)
• Image references use digests (@sha256:...) not mutable tags
• Base images are regularly updated to include security patches

Build Security

• Dockerfiles use a non-root USER directive
• .dockerignore excludes .env, .git, node_modules, and other sensitive files
• Build arguments do not contain secrets (use BuildKit secret mounts instead)
• Layer caching does not leak sensitive data between stages
• HEALTHCHECK is defined for orchestrator integration

Runtime Security

• All containers run as non-root (runAsNonRoot: true)
• All capabilities are dropped (capabilities.drop: [ALL])
• Root filesystem is read-only (readOnlyRootFilesystem: true)
• Seccomp profile is set to RuntimeDefault or a custom profile
• Resource requests and limits are defined for all containers
• Service account tokens are not auto-mounted unless required

Kubernetes Security

• PodSecurityStandards are enforced at restricted or baseline level
• Default-deny NetworkPolicies are applied in every namespace
• RBAC follows least privilege -- no broad ClusterRole bindings
• Admission controllers validate configurations before deployment
• Secrets are managed externally (GCP Secret Manager, Vault, ExternalSecrets)
• Pod disruption budgets ensure availability during security updates
• Runtime monitoring (Falco or equivalent) is deployed

Related Skills

• @cis-benchmarks -- CIS Docker and Kubernetes Benchmark automated scanning
• @cloud-security-patterns -- cloud-native security architectures for GCP, AWS, Azure
• @owasp-top10 -- application-layer security for containerized web applications

Additional Resources

• NIST SP 800-190 -- Application Container Security Guide
• Kubernetes Pod Security Standards -- official PSS documentation
• Docker Security Best Practices -- official Docker guidance
• Kubernetes Network Policies -- NetworkPolicy reference
• Falco Runtime Security -- open-source container runtime threat detection
• cosign -- container image signing and verification
• Trivy -- comprehensive container vulnerability scanner
• GKE Security Overview -- GKE-specific security features