Docker Security Knowledge Base

Quick reference for Docker security hardening patterns in PHP applications.

Security Layers

+---------------------------------------------------------------------------+
|                     DOCKER SECURITY LAYERS                                 |
+---------------------------------------------------------------------------+
|                                                                            |
|   Layer 1: Build Security                                                  |
|   +--------------------------------------------------------------------+  |
|   | Minimal base image | Pinned versions | No secrets in layers        |  |
|   | Multi-stage builds | .dockerignore   | Verified base images        |  |
|   +--------------------------------------------------------------------+  |
|                                                                            |
|   Layer 2: Image Security                                                  |
|   +--------------------------------------------------------------------+  |
|   | Vulnerability scanning | SBOM generation | Signed images            |  |
|   | No unnecessary packages | Read-only layers | Minimal attack surface |  |
|   +--------------------------------------------------------------------+  |
|                                                                            |
|   Layer 3: Runtime Security                                                |
|   +--------------------------------------------------------------------+  |
|   | Non-root user      | Read-only filesystem | Dropped capabilities   |  |
|   | Resource limits     | No privileged mode   | Seccomp profiles       |  |
|   +--------------------------------------------------------------------+  |
|                                                                            |
|   Layer 4: Network Security                                                |
|   +--------------------------------------------------------------------+  |
|   | Network segmentation | Encrypted traffic | Internal DNS only        |  |
|   | No host networking   | Firewall rules    | Service mesh (optional)  |  |
|   +--------------------------------------------------------------------+  |
|                                                                            |
|   Layer 5: Secrets Management                                              |
|   +--------------------------------------------------------------------+  |
|   | Docker secrets     | Build secrets      | External vault            |  |
|   | No ENV for secrets | Encrypted at rest  | Rotation policies         |  |
|   +--------------------------------------------------------------------+  |
|                                                                            |
+---------------------------------------------------------------------------+

Non-Root User Patterns

Alpine-based Image

# syntax=docker/dockerfile:1
FROM php:8.4-fpm-alpine

# Create non-root user and group
RUN addgroup -g 1000 -S app && \
    adduser -u 1000 -S app -G app -h /home/app -s /bin/sh

# Set working directory with correct ownership
WORKDIR /var/www/html
RUN chown -R app:app /var/www/html

# Switch to non-root user
USER app

Debian-based Image

FROM php:8.4-fpm

RUN groupadd -g 1000 app && \
    useradd -u 1000 -g app -m -s /bin/bash app

WORKDIR /var/www/html
RUN chown -R app:app /var/www/html

USER app

Secrets Management

Build Secrets (BuildKit)

# syntax=docker/dockerfile:1
FROM composer:2 AS deps

# Use build secret for private repository access
RUN --mount=type=secret,id=composer_auth,target=/root/.composer/auth.json \
    composer install --no-dev --no-scripts --prefer-dist

FROM php:8.4-fpm-alpine AS production
COPY --from=deps /app/vendor /var/www/html/vendor
# Secret is NOT present in final image

# Build with secret
DOCKER_BUILDKIT=1 docker build \
    --secret id=composer_auth,src=auth.json \
    -t myapp:latest .

Runtime Secrets (Docker Compose)

services:
  php:
    image: myapp:latest
    secrets:
      - db_password
      - app_key

secrets:
  db_password:
    file: ./secrets/db_password.txt
  app_key:
    file: ./secrets/app_key.txt

<?php
// Read secret at runtime
function getSecret(string $name): string
{
    $path = '/run/secrets/' . $name;
    if (!file_exists($path)) {
        throw new \RuntimeException("Secret not found: {$name}");
    }
    return trim(file_get_contents($path));
}

Docker Swarm Secrets

# Create encrypted secret
echo "s3cur3p@ss" | docker secret create db_password -

# Use in service
docker service create \
    --secret db_password \
    --name php-app myapp:latest

Image Scanning Tools

Quick Scan Commands

# Trivy - comprehensive scan
trivy image --severity HIGH,CRITICAL myapp:latest

# Grype - fast vulnerability scan
grype myapp:latest --fail-on high

# Docker Scout - built-in scanning
docker scout cves myapp:latest
docker scout recommendations myapp:latest

Capability Management

# docker-compose.yml
services:
  php:
    cap_drop:
      - ALL
    cap_add:
      - NET_BIND_SERVICE    # Bind to ports < 1024 (if needed)
      - CHOWN               # Change file ownership (if needed)
    security_opt:
      - no-new-privileges:true

Minimal Capabilities for PHP-FPM

Read-Only Filesystem

services:
  php:
    read_only: true
    tmpfs:
      - /tmp:size=64M
      - /var/run:size=1M
      - /var/log:size=32M
    volumes:
      - php-sessions:/var/lib/php/sessions

Network Policies

services:
  nginx:
    networks:
      - frontend
      - backend

  php:
    networks:
      - backend          # No direct external access

  postgres:
    networks:
      - backend          # Database isolated

networks:
  frontend:
    driver: bridge
  backend:
    driver: bridge
    internal: true       # No external access

OWASP Docker Top 10

Detection Patterns

# Find Dockerfiles with security issues
Grep: "FROM.*:latest" --glob "**/Dockerfile*"
Grep: "USER root" --glob "**/Dockerfile*"
Grep: "privileged.*true" --glob "**/docker-compose*.yml"
Grep: "ENV.*PASSWORD|ENV.*SECRET|ENV.*KEY" --glob "**/Dockerfile*"
Grep: "cap_add.*ALL|cap_add.*SYS_ADMIN" --glob "**/docker-compose*.yml"

# Check for proper security measures
Grep: "USER app|USER www-data|USER nobody" --glob "**/Dockerfile*"
Grep: "cap_drop.*ALL" --glob "**/docker-compose*.yml"
Grep: "read_only.*true" --glob "**/docker-compose*.yml"
Grep: "no-new-privileges" --glob "**/docker-compose*.yml"

References

For detailed information, load these reference files:

• references/security-checklist.md — Comprehensive security checklist with 50+ items