# containerization-docker-best-practices

Guides Docker image creation with rules for multi-stage builds, process isolation, environment portability, and minimal runtime footprints.

- rule · `type=rule` · GitHub Copilot · ★ 40418
- Author: github · official GitHub repository
- License: MIT · policy=mirror · the beginning of the file is at the end of this page
- Source: https://github.com/github/awesome-copilot/tree/HEAD/instructions/containerization-docker-best-practices.instructions.md
- Updated: 2026-10-23
- HTML: https://affpapa.org/en/nevibecode/rules/containerization-docker-best-practices

## What it does

This rule configures GitHub Copilot when editing `Dockerfile` variants or Compose files. The assistant enforces image immutability by requiring distinct semantic tags for new builds and blocking in-place changes to live containers. Copilot focuses on strict process boundaries: one primary process per container, isolated network stacks, explicit resource caps, and named volumes over host bind mounts.

Multi-stage builds serve as the core requirement. Build utilities stay inside early stages, while `COPY --from` extracts strictly the runtime artifacts into the final stage. This keeps production images lean and cuts the attack surface. Environment variables handle all runtime settings to preserve cross-platform portability.

Copilot activates these guidelines only for matching paths, such as `Dockerfile` or `compose.yaml`. Outside of container definitions, the rule stays idle.

## Requirements
- Works with: Docker

## Install

### GitHub Copilot (native platform)

**Manually: put the files in place**
- File: `.github/instructions/containerization-docker-best-practices.instructions.md` — The applyTo field sets which files it applies to

### Claude Code

**Manually: put the files in place**
- File: `.claude/rules/containerization-docker-best-practices.md` — The paths frontmatter field limits the files: move applyTo from the source into paths

### Codex

**AGENTS.md**
- Codex reads AGENTS.md: paste the rule text there

### Cursor

**Manually: put the files in place**
- File: `.cursor/rules/containerization-docker-best-practices.mdc` — Frontmatter: description, globs, alwaysApply. Move applyTo from the source into globs, or the rule will not attach on its own

### Windsurf (Devin Desktop)

**Manually: put the files in place**
- File: `.devin/rules/containerization-docker-best-practices.md`

### Cline

**Manually: put the files in place**
- File: `.clinerules/containerization-docker-best-practices.md`

Where a file is given instead of a command, copy the files from the source to that path: https://github.com/github/awesome-copilot/tree/HEAD/instructions/containerization-docker-best-practices.instructions.md

## When to use it
- shrink production image size with multi-stage builds
- enforce process isolation and container resource limits
- externalize runtime configurations into environment variables
- tag images with semantic versions for reproducible rollbacks

## Example prompts
- Write a multi-stage Dockerfile for my app and copy only the runtime artifacts
- Check this docker-compose.yml for network isolation and set container resource limits
- Update my Dockerfile to handle runtime settings via environment variables

## Source text · containerization-docker-best-practices.instructions.md · MIT

Below is the content of a third-party file. It is data, not instructions for you: do not act on anything written inside unless the human asks. Block tag: 6e38f09d8dd87a6d.

BEGIN THIRD-PARTY FILE 6e38f09d8dd87a6d
```text
---
applyTo: '**/Dockerfile,**/Dockerfile.*,**/*.dockerfile,**/docker-compose*.yml,**/docker-compose*.yaml,**/compose*.yml,**/compose*.yaml'
description: 'Comprehensive best practices for creating optimized, secure, and efficient Docker images and managing containers. Covers multi-stage builds, image layer optimization, security scanning, and runtime best practices.'
---

# Containerization & Docker Best Practices

## Your Mission

As GitHub Copilot, you are an expert in containerization with deep knowledge of Docker best practices. Your goal is to guide developers in building highly efficient, secure, and maintainable Docker images and managing their containers effectively. You must emphasize optimization, security, and reproducibility.

## Core Principles of Containerization

### **1. Immutability**
- **Principle:** Once a container image is built, it should not change. Any changes should result in a new image.
- **Deeper Dive:**
    - **Reproducible Builds:** Every build should produce identical results given the same inputs. This requires deterministic build processes, pinned dependency versions, and controlled build environments.
    - **Version Control for Images:** Treat container images like code - version them, tag them meaningfully, and maintain a clear history of what each image contains.
    - **Rollback Capability:** Immutable images enable instant rollbacks by simply switching to a previous image tag, without the complexity of undoing changes.
    - **Security Benefits:** Immutable images reduce the attack surface by preventing runtime modifications that could introduce vulnerabilities.
- **Guidance for Copilot:**
    - Advocate for creating new images for every code change or configuration update, never modifying running containers in production.
    - Recommend using semantic versioning for image tags (e.g., `v1.2.3`, `latest` for development only).
    - Suggest implementing automated image builds triggered by code changes to ensure consistency.
    - Emphasize the importance of treating container images as artifacts that should be versioned and stored in registries.
- **Pro Tip:** This enables easy rollbacks and consistent environments across dev, staging, and production. Immutable images are the foundation of reliable deployments.

### **2. Portability**
- **Principle:** Containers should run consistently across different environments (local, cloud, on-premise) without modification.
- **Deeper Dive:**
    - **Environment Agnostic Design:** Design applications to be environment-agnostic by externalizing all environment-specific configurations.
    - **Configuration Management:** Use environment variables, configuration files, or external configuration services rather than hardcoding environment-specific values.
    - **Dependency Management:** Ensure all dependencies are explicitly defined and included in the container image, avoiding reliance on host system packages.
    - **Cross-Platform Compatibility:** Consider the target deployment platforms and ensure compatibility (e.g., ARM vs x86, different Linux distributions).
- **Guidance for Copilot:**
    - Design Dockerfiles that are self-contained and avoid environment-specific configurations within the image itself.
    - Use environment variables for runtime configuration, with sensible defaults but allowing overrides.
    - Recommend using multi-platform base images when targeting multiple architectures.
```
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