Docker

When to Use

Use when the task involves Docker, Dockerfiles, container builds, Compose, image publishing, networking, volumes, logs, debugging, or production container operations. This skill is stateless and should be applied directly whenever Docker work appears.

Quick Reference

Core Rules

1. Pin Image Versions

• python:3.11.5-slim not python:latest
• Today's latest differs from tomorrow's — breaks immutable builds

2. Combine RUN Commands

• apt-get update && apt-get install -y pkg in ONE layer
• Separate layers = stale package cache weeks later

3. Non-Root by Default

• Add USER nonroot in Dockerfile
• Running as root fails security scans and platform policies

4. Set Resource Limits

• -m 512m on every container
• OOM killer strikes without warning otherwise

5. Configure Log Rotation

• Default json-file driver has no size limit
• One chatty container fills disk and crashes host

Image Traps

• Multi-stage builds: forgotten --from=builder copies from wrong stage silently
• COPY before RUN invalidates cache on every file change — copy requirements first, install, then copy code
• ADD extracts archives automatically — use COPY unless you need extraction
• Build args visible in image history — never use for secrets

Runtime Traps

• localhost inside container is container's localhost — bind to 0.0.0.0
• Port already in use: previous container still stopping — wait or force remove
• Exit code 137 = OOM killed, 139 = segfault — check with docker inspect --format='{{.State.ExitCode}}'
• No shell in distroless images — docker cp files out or use debug sidecar

Networking Traps

• Container DNS only works on custom networks — default bridge can't resolve names
• Published ports bind to 0.0.0.0 — use 127.0.0.1:5432:5432 for local-only
• Zombie connections from killed containers — set health checks and restart policies

Compose Traps

• depends_on waits for container start, not service ready — use condition: service_healthy
• .env file in wrong directory silently ignored — must be next to docker-compose.yml
• Volume mounts overwrite container files — empty host dir = empty container dir
• YAML anchors don't work across files — use multiple compose files instead

Volume Traps

• Anonymous volumes accumulate silently — use named volumes
• Bind mounts have permission issues — container user must match host user
• docker system prune doesn't remove named volumes — add --volumes flag
• Stopped container data persists until container removed

Resource Leaks

• Dangling images grow unbounded — docker image prune regularly
• Build cache grows forever — docker builder prune reclaims space
• Stopped containers consume disk — docker container prune or --rm on run
• Networks pile up from compose projects — docker network prune

Secrets and Security

• ENV and COPY bake secrets into layer history permanently — use secrets mount or runtime env
• --privileged disables all security — almost never needed, find specific capability instead
• Images from unknown registries may be malicious — verify sources
• Build args visible in image history — don't use for secrets

Debugging

• Exit code 137 = OOM killed, 139 = segfault — check docker inspect --format='{{.State.ExitCode}}'
• Container won't start: check logs even for failed containers — docker logs <container>
• No shell in distroless images — docker cp files out or use debug sidecar
• Inspect filesystem of dead container — docker cp deadcontainer:/path ./local

Related Skills

Install with clawhub install <slug> if user confirms:

• devops — deployment pipelines
• linux — host system management
• server — server administration

Feedback

• If useful: clawhub star docker
• Stay updated: clawhub sync