Docker

docker

When to use

Use this skill when working with Docker configuration, container setup, Dockerfile changes, or docker-compose modifications.

Do NOT use when:

• Production deployment (use aws-infrastructure skill)
• Codespaces setup (use devcontainer skill)

Procedure: Modify Docker setup

1. Gather context — read project Docker docs in agents/ or Docs/, check Makefile/Taskfile.yml for targets, read docker-compose.yml/compose.yaml for service layout.
2. Identify scope — determine which service(s) are affected (PHP, NGINX, worker, scheduler, database).
3. Inspect current state — run docker compose ps to see running containers and their health status.
4. Make the change — edit the relevant file (Dockerfile, compose file, NGINX config, Makefile target). Follow the conventions in the reference sections below.
5. Rebuild affected containers — docker compose build <service> (add --no-cache if Dockerfile base layers changed).
6. Verify — docker compose up -d, check docker compose ps for healthy status, run a smoke test (e.g., make test-quick or curl localhost).

Project architecture

Dockerfile (.docker/Dockerfile)

Multi-stage build with these targets:

Stage	Purpose
base	Alpine + PHP-FPM + system packages + extensions
dev	Development: Xdebug, dev tools, Composer dev deps
pro	Production: optimized, no dev deps, New Relic agent

Key build args:

• PHP_VERSION — extracted from Dockerfile, used by CI
• COMPOSER_AUTH — private registry access (passed as secret)
• CACHEBUST — weekly cache invalidation (date +%Y-%U)
• COMPOSER_NO_DEV — 1 for production, 0 for dev

Dual-container architecture (PHP projects)

Some projects run two PHP-FPM containers simultaneously (fast + Xdebug):

Container	Purpose	PHP-FPM mode
{project}-php	Fast execution, no debugger	pm = dynamic
{project}-php-xdebug	Xdebug enabled, debugging	pm = ondemand

NGINX routes requests based on HTTP headers:

• No header → fast container
• X-Xdebug-Enable: 1 or X-Debug-Session: PHPSTORM → Xdebug container

docker-compose services

Read docker-compose.yml / compose.yaml to discover the actual service names. Common patterns:

Service type	Description
PHP-FPM	Main application server
PHP-FPM + Xdebug	Debugging container
NGINX	Reverse proxy
Queue worker	Background job processing (e.g., Horizon)
Scheduler	Cron/task scheduler
Database	MariaDB / MySQL / PostgreSQL
Cache	Redis / Memcached

Conventions

Container commands

• Always execute PHP commands inside the container, never on the host.
• Use docker compose exec -T <service> ... for non-interactive (scripts, CI).
• Use make console for interactive shell access.
• Use make console-xdebug for Xdebug container access.

Image building

• Production images use target: pro — no dev dependencies.
• Check the project's CI/CD config for target platform and registry.
• Docker Hub login may be needed for pulling base images (rate limits).

PHP extensions

Extensions are installed via mlocati/php-extension-installer:

• Check the Dockerfile for the current list.
• Add new extensions in the base stage so they're available in all targets.

Environment files

• .env is NOT baked into the Docker image.
• Production: .env is fetched from AWS Secrets Manager at deploy time.
• Development: .env is mounted via docker-compose volumes.

Makefile targets

Always check the Makefile for available targets before using raw docker commands:

make start              # Start all containers
make stop               # Stop all containers
make console            # Enter PHP container (bash)
make console-xdebug     # Enter Xdebug PHP container
make composer-install   # Run composer install in container
make migrate            # Run migrations
make migrate-and-seed   # Run migrations + seed
make test               # Run all tests (parallel)

Container orchestration

Environment synchronization

When the development environment is out of sync (missing containers, wrong state):

1. Check status — docker compose ps to see which services are running.
2. Start missing services — make start or docker compose up -d.
3. Rebuild if needed — docker compose build --no-cache <service> after Dockerfile changes.
4. Reset state — make migrate-and-seed after fresh container start.

Common sync issues

Symptom	Cause	Fix
"Connection refused"	Container not running	make start
"Table not found"	Migrations not run	make migrate-and-seed
"Class not found"	Composer not installed	make composer-install
Old PHP version	Image not rebuilt	docker compose build <php-service>
Extension missing	Dockerfile changed	Rebuild with --no-cache

Multi-project orchestration

When running multiple projects simultaneously:

• Check for port conflicts — each project needs unique exposed ports.
• Use Traefik (see traefik skill) for routing by domain instead of port.
• Shared services (MariaDB, Redis) can be in a dedicated docker-compose.shared.yml.

Security hardening checklist

When creating or reviewing Dockerfiles:

• Non-root user — create user with specific UID/GID, use USER directive before CMD.
• No secrets in layers — never ENV or COPY secrets. Use --mount=type=secret (BuildKit) or runtime secrets.
• Minimal packages — only install what's needed. Remove package manager cache in the same RUN layer.
• Read-only root filesystem — use --read-only flag where possible, mount writable dirs explicitly.
• No latest tag — pin base image versions (node:18.19-alpine, not node:latest).
• Scan images — use docker scout quickview or Trivy for vulnerability scanning.

# Security pattern
RUN addgroup -g 1001 -S appgroup && \
    adduser -S appuser -u 1001 -G appgroup
COPY --chown=appuser:appgroup . .
USER 1001

Health check patterns

Always add health checks to long-running services:

HEALTHCHECK --interval=30s --timeout=10s --start-period=5s --retries=3 \
  CMD curl -f http://localhost:8080/health || exit 1

In docker-compose, use condition: service_healthy for dependency ordering:

services:
  app:
    depends_on:
      db:
        condition: service_healthy

Image size optimization

Technique	Impact	When
Multi-stage builds	High	Always — separate build from runtime
Alpine base images	High	When compatibility allows
Distroless images	High	Production, no shell needed
.dockerignore	Medium	Always — exclude node_modules, .git, tests, docs
Combine RUN layers	Medium	When installing packages + cleaning cache
Copy only artifacts	Medium	COPY --from=build only what's needed

Build cache optimization

Use BuildKit cache mounts for package managers:

# Composer (PHP)
RUN --mount=type=cache,target=/root/.composer/cache \
    composer install --no-dev --optimize-autoloader

# npm (Node.js)
RUN --mount=type=cache,target=/root/.npm \
    npm ci --only=production

Layer ordering for cache efficiency:

1. System packages (changes rarely)
2. Dependency files (composer.json, package.json) — changes sometimes
3. RUN install — cached if dependency files unchanged
4. Source code (COPY . .) — changes often, last layer

Output format

1. Modified Docker configuration files (Dockerfile, docker-compose.yml)
2. Updated Makefile targets if applicable
3. Rebuild/restart instructions for affected containers

Auto-trigger keywords

• Docker
• docker-compose
• container
• Dockerfile
• PHP container

Known pitfalls

Symptom	Root cause	Fix
Every build reinstalls all dependencies (builds are slow)	COPY . . runs before the dependency install, so any source edit busts the dependency layer's cache	Copy only the manifest + lockfile (composer.json+composer.lock / package.json+lock), install deps, THEN COPY . .
Image is much larger / slower to push than expected	No .dockerignore, so .git, vendor/, node_modules/, and local env files enter the build context and image	Add a .dockerignore excluding VCS, installed deps, build output, and secrets
vendor/ or node_modules/ is empty inside the container even though install ran	A bind-mount of the project directory shadows the image's installed-deps directory	Put a named/anonymous volume over the deps dir, or don't bind-mount over it
Files the container writes are owned by root on the host	The container process runs as UID 0; bind-mounted writes inherit that owner	Run as a non-root USER whose UID matches the host user, or chown on entry
Container exits immediately with code 0	The CMD process daemonizes/backgrounds, so PID 1 has nothing to keep alive	Run the long-lived process in the foreground as PID 1 (no &, no daemonize flag)

Gotcha

• All PHP commands (artisan, composer, phpunit) must run INSIDE the PHP container — never on the host.
• The fast container and Xdebug container share the same codebase but have different PHP configs — don't confuse them.
• docker compose down -v destroys volumes including the database — use down without -v unless you mean it.
• The model forgets to use docker compose exec -T (no TTY) when running in scripts or CI.

Do NOT

• Do NOT change the base Alpine or PHP version without checking CI compatibility.
• Do NOT add dev-only tools to the pro stage.
• Do NOT hardcode secrets in the Dockerfile — use build args or runtime secrets.
• Do NOT change platform without verifying AWS runner architecture.

Related

• Skill: traefik — local reverse proxy with real domains and HTTPS
• Skill: devcontainer — DevContainer and Codespaces setup
• Skill: php-debugging — Xdebug dual-container architecture
• Rule: docker-commands.md — all PHP commands run inside Docker