Docker Compose Patterns Agent Skill

Docker Compose Patterns

Overview

This skill provides rules for creating, reviewing, and debugging Docker Compose configurations. Use it when the main artifact is compose.yaml or compose.override.yaml and the task is about service wiring rather than image-build internals.

When to use this skill

Activate this skill when:

• Creating a new compose.yaml for a project
• Adding or modifying services in an existing Compose file
• Setting up development overrides with compose.override.yaml
• Debugging service startup ordering or connectivity issues

Do not use this skill when

Do not use this skill when:

• The project has no Docker setup yet and the main need is an initial scaffold
• The main task is writing or optimizing a Dockerfile
• The main task is improving build caching, image size, or runtime user configuration

Core guidance

File naming

Use compose.yaml as the canonical filename. Do not use docker-compose.yml or docker-compose.yaml — those are legacy names.

Service definitions

• Give services clear, lowercase names that reflect their role: web, db, cache, worker.
• Always pin image tags to a specific version. Never use latest or omit the tag.
• Set restart: unless-stopped for long-running infrastructure services and non-development deployments.
• Add container_name only when external tools need a predictable name. Otherwise, let Compose generate names.

Dependency modeling

• Use depends_on with condition: service_healthy for services that must be ready before dependents start.
• Every service listed in depends_on with a health condition must have a healthcheck defined.
• Do not rely on depends_on without conditions — it only guarantees container start, not readiness.

Health checks

• Always add a healthcheck to database services (Postgres, MySQL, Redis, MongoDB).
• Use the service's native client tool for health checks when available (e.g., pg_isready, redis-cli ping, mysqladmin ping).
• Set reasonable interval, timeout, retries, and start_period values. Start with: interval: 5s, timeout: 3s, retries: 3, start_period: 10s.

Health checks for distroless or scratch images

Distroless, scratch-based, and hardened images contain no shell, curl, or wget. Do not bake tools into these images — that defeats their purpose. Instead, use a healthcheck sidecar that shares the application's network namespace:

services:
  api:
    build:
      context: .
      target: runtime          # distroless / hardened image
    ports:
      - "8080:8080"
    # No healthcheck here — the image has no tools to run one

  api-health:
    image: curlimages/curl:8
    network_mode: "service:api"   # shares api's localhost
    entrypoint: ["sleep", "infinity"]  # keep sidecar alive for healthcheck
    healthcheck:
      test: ["CMD", "curl", "-f", "http://localhost:8080/health"]
      interval: 30s
      timeout: 5s
      retries: 3
      start_period: 45s
    deploy:
      resources:
        limits:
          memory: 32M

Key points:

• The sidecar must stay alive with entrypoint: ["sleep", "infinity"] so Compose can execute the healthcheck inside it.
• network_mode: "service:api" makes localhost inside the sidecar resolve to the api container's loopback — no extra networking needed.
• Keep the sidecar lightweight with a resource limit (32MB is sufficient for curl).
• Services that depend on api being ready should reference the sidecar, not the api directly:

  worker:
    depends_on:
      api-health:
        condition: service_healthy

Volumes

• Use named volumes for data that must persist across container recreations (database data, uploaded files).
• Use bind mounts only for development-time source code syncing.
• Define all named volumes in the top-level volumes: key.
• Do not mount the Docker socket unless the service genuinely requires it.

Networks

• For single-application stacks, the default network is sufficient. Do not create custom networks unless you need isolation between service groups.
• When creating custom networks, prefer bridge driver and give networks descriptive names.
• Use the top-level networks: key to define all custom networks.

Environment variables

• Use environment: for non-sensitive values that are few in number.
• Use env_file: pointing to a .env file for longer lists of variables.
• Never hardcode secrets (passwords, API keys) directly in compose.yaml. Use env_file: or Docker secrets.
• When defaults are needed in the environment: block for local development, use variable substitution with fallbacks: ${DB_PASSWORD:-postgres}. Never write bare plaintext values for password fields.
• Add .env to .gitignore.

Development overrides

• Use compose.override.yaml for development-only settings. Compose loads it automatically alongside compose.yaml.
• Put bind mounts for source code, debug ports, and development environment variables in the override file.
• Use develop.watch for file-syncing and auto-rebuild in development when supported.
• Keep production-oriented settings in the base compose.yaml and override only what changes for development.

Compose Watch

• Prefer develop.watch over manual bind mounts for development workflows.
• Use action: sync for files that should be copied into the container on change (source code).
• Use action: rebuild for files that require a full image rebuild (dependency files like package.json, requirements.txt).
• Use action: sync+restart for configuration files that need a process restart.

Destructive commands

Some Compose commands delete data irreversibly. Before running any of the following, state exactly which data will be deleted and get explicit confirmation from the user — do not run them as a side effect of debugging, restarting, or "cleaning up" a stack:

• docker compose down -v / docker compose down --volumes — deletes named volumes, including database data.
• docker volume rm / docker volume prune run against a Compose project's volumes — deletes volumes directly. For the standalone case (no Compose project in play), see docker-destructive-guardrails instead. A volume referenced via external: true isn't managed by the Compose project either (down -v won't touch it) — treat it as the standalone case too: run docker volume rm without -f first, and get explicit confirmation before deleting it.
• docker compose rm -v — deletes anonymous volumes attached to removed containers.

If the goal is only to restart services or reclaim containers/networks, use docker compose down (no -v) or docker compose restart instead — these leave named volumes intact.

Related skills

• For first-time Docker project scaffolding and baseline file creation, use docker-project-foundations.
• For Dockerfile internals, build caching, multi-stage builds, and .dockerignore, use docker-build-strategies.
• For destructive Docker CLI commands outside Compose (docker system prune, docker rm -f, image/network/builder pruning, standalone volume deletion) and a cross-product index of destructive-command guardrails, use docker-destructive-guardrails.

References

• references/service-dependencies.md — Detailed guidance on depends_on, health check patterns for common databases, and startup ordering strategies.
• references/volumes-and-networks.md — Patterns for volume mounts, named volumes, bind mounts, and network configuration.

Assets

• assets/compose-web-app.yaml — Complete multi-service web app (app + Postgres + Redis) with health checks, dependencies, and named volumes.
• assets/compose-dev-override.yaml — Development override showing bind mounts, debug ports, and Compose Watch configuration.
• assets/bad-vs-good.md — Before/after comparisons of common Compose mistakes and their fixes.

Scripts

• scripts/verify-compose.sh — Validates compose.yaml with docker compose config --quiet, without printing resolved configuration.

  bash scripts/verify-compose.sh [--help]

Exit status is 0 when the Compose configuration is valid or help is requested, the non-zero status from docker compose config --quiet when validation fails, and 2 for invalid arguments. Plain docker compose config can expose interpolated and env_file credentials in tool output or logs; use quiet validation by default. Compose warnings and errors are still emitted and may contain sensitive details.

Checks

• checks/verification.md — Detailed verification runbook for manual review.