docker

name	docker
description	Apply when writing or reviewing Dockerfiles, docker compose files, or container build pipelines. Covers layer caching, multi-stage builds, security hardening, and compose conventions.
license	MIT
version	1.0.0
tokens_target	1700
triggers	["dockerfile","docker compose","container build"]
loads_after	["code-quality"]
supersedes	[]

Sub-Skill: Docker / Container Conventions

Purpose: Prevent common container mistakes — busted layer caches, bloated images, insecure defaults, and compose misconfigurations — before they reach CI or production.

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Rules

Layer Ordering & Cache

1. Dependency layer first. Always install dependencies in a separate layer before copying application source, so a source change does not invalidate the package cache. Reference: ERR-2026-020
2. Copy only what's needed early. Before copying the full source tree, copy only the dependency manifest files (e.g., requirements.txt, package.json, pyproject.toml) so the install layer is cached independently.
3. Minimise layer count. Prefer chaining related RUN commands with && and \ continuations rather than issuing one RUN per command; each RUN creates a new layer.
4. Order by change frequency. Always place instructions that change rarely (OS packages, global tools) before instructions that change often (app source, config files).

Multi-Stage Builds

5. Use multi-stage for compiled artefacts. Always use a builder stage to compile or bundle, then copy only the final artefact into a minimal runtime stage; never ship build toolchains in the production image.
6. Name every stage. Use AS <name> on every FROM line so later stages and docker build --target calls are readable and stable.
7. Pin the runtime base image. Always pin base images to a specific digest or immutable tag (e.g., python:3.12.3-slim) in the runtime stage; never use latest in production Dockerfiles.

Security

8. Run as non-root. Always create a dedicated non-root user and switch to it with USER before the final CMD/ENTRYPOINT; never run application processes as root inside a container.
9. Never embed secrets in image layers. Never pass secrets via ARG or ENV in a Dockerfile; use Docker BuildKit --secret mounts or runtime environment injection instead.
10. Scan images before push. Before pushing any image to a registry, run an image vulnerability scanner (e.g., trivy image, docker scout) and fail the pipeline on critical CVEs.
11. Prefix docker run -v from Git Bash with MSYS_NO_PATHCONV=1. Always prefix docker run -v calls issued from Git Bash or MSYS on Windows with MSYS_NO_PATHCONV=1 to prevent path mangling. Reference: ERR-2026-013

.dockerignore & Context

12. Maintain a .dockerignore. Always keep a .dockerignore at the repo root that excludes .git, test fixtures, local env files, and build artefacts; a large build context slows every build and may leak secrets.

Health Checks & Signal Handling

13. Declare a HEALTHCHECK. Always add a HEALTHCHECK instruction so orchestrators (Compose, Kubernetes) can detect unhealthy containers without external probes.
14. Use exec-form ENTRYPOINT. Always write ENTRYPOINT in exec form (["executable", "arg"]) rather than shell form so the process receives OS signals directly and docker stop terminates it cleanly.

Compose Conventions

15. Set resource limits in compose. Always declare deploy.resources.limits (CPU and memory) for every service in docker-compose.yml to prevent a runaway container from starving the host.
16. Use compose profiles for optional services. Prefer assigning optional services (e.g., observability stacks, seed jobs) to named profiles so docker compose up starts only the core services by default.
17. Isolate networks per stack. Avoid using the default bridge network across unrelated stacks; define explicit named networks and attach only the services that need to communicate.

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See also

• skills/code-quality/SKILL.md — general layering and change-frequency ordering principles
• skills/review-deployment/SKILL.md — deployment checklist that includes image scanning and migration ordering

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Notes

• ERR-2026-020 is the canonical reference for layer-cache busting caused by copying source before installing dependencies.
• ERR-2026-013 covers MSYS path mangling on Windows when using docker run -v.