Docker Management

Docker Management

Manage Docker containers, images, volumes, networks, and Compose stacks using standard Docker CLI commands. No additional dependencies beyond Docker Engine and Docker Compose v2.

When to Use

Activate this skill when the user requests any of the following:

• Container lifecycle — run, stop, start, restart, remove, pause/unpause containers
• Container interaction — exec into containers, copy files, tail logs, inspect, view stats
• Image management — build, pull, push, tag, remove, inspect, save/load images
• Docker Compose — up, down, ps, logs, exec, build, config validation for multi-service stacks
• Volumes & networks — create, inspect, remove, prune, connect/disconnect containers
• Troubleshooting — analyze crashing containers, exit codes, resource limits, log errors
• Disk cleanup — check Docker disk usage, prune dangling/unused resources
• Dockerfile review — optimize layer ordering, multi-stage builds, .dockerignore, image size

Trigger keywords: docker run, docker compose, docker build, container, image, volume, network, docker logs, docker exec, docker prune, dockerfile, docker system df.

Prerequisites

• Docker Engine installed and running
• User added to the docker group (Linux/macOS) or Docker Desktop running (Windows)
• Docker Compose v2 (included with modern Docker installations)

Quick check:

docker --version && docker compose version

On Windows PowerShell, also verify the Docker Desktop daemon is running:

docker info

If docker info fails, start Docker Desktop and wait for the whale icon to become steady.

Procedure

1. Identify the domain

Determine which area the request falls into before running commands:

• Container lifecycle → run, stop, start, restart, rm, pause/unpause
• Container interaction → exec, cp, logs, inspect, stats
• Image management → build, pull, push, tag, rmi, save/load
• Docker Compose → up, down, ps, logs, exec, build, config
• Volumes & networks → create, inspect, rm, prune, connect
• Troubleshooting → log analysis, exit codes, resource issues

2. Container operations

Run a new container:

# Detached service with port mapping
docker run -d --name web -p 8080:80 nginx

# With environment variables
docker run -d -e POSTGRES_PASSWORD=secret -e POSTGRES_DB=mydb --name db postgres:16

# With persistent data (named volume)
docker run -d -v pgdata:/var/lib/postgresql/data --name db postgres:16

# For development (bind mount source code)
docker run -d -v $(pwd)/src:/app/src -p 3000:3000 --name dev my-app

# Interactive debugging (auto-remove on exit)
docker run -it --rm ubuntu:22.04 /bin/bash

# With resource limits and restart policy
docker run -d --memory=512m --cpus=1.5 --restart=unless-stopped --name app my-app

Windows PowerShell note: Replace $(pwd) with ${PWD} in PowerShell. Example:

docker run -d -v ${PWD}/src:/app/src -p 3000:3000 --name dev my-app

Key flags: -d detached, -it interactive+tty, --rm auto-remove, -p port (host:container), -e env var, -v volume, --name name, --restart restart policy.

Manage running containers:

docker ps                        # running containers
docker ps -a                     # all (including stopped)
docker stop NAME                 # graceful stop
docker start NAME                # start stopped container
docker restart NAME              # stop + start
docker rm NAME                   # remove stopped container
docker rm -f NAME                # force remove running container
docker container prune           # remove ALL stopped containers

Interact with containers:

docker exec -it NAME /bin/sh          # shell access (use /bin/bash if available)
docker exec NAME env                   # view environment variables
docker exec -u root NAME apt update    # run as specific user
docker logs --tail 100 -f NAME         # follow last 100 lines
docker logs --since 2h NAME            # logs from last 2 hours
docker cp NAME:/path/file ./local      # copy file from container
docker cp ./file NAME:/path/           # copy file to container
docker inspect NAME                    # full container details (JSON)
docker stats --no-stream               # resource usage snapshot
docker top NAME                        # running processes

3. Image management

# Build
docker build -t my-app:latest .
docker build -t my-app:prod -f Dockerfile.prod .
docker build --no-cache -t my-app .              # clean rebuild
DOCKER_BUILDKIT=1 docker build -t my-app .       # faster with BuildKit

# Pull and push
docker pull node:20-alpine
docker login ghcr.io
docker tag my-app:latest registry/my-app:v1.0
docker push registry/my-app:v1.0

# Inspect
docker images                          # list local images
docker history IMAGE                   # see layers
docker inspect IMAGE                   # full details

# Cleanup
docker image prune                     # remove dangling (untagged) images
docker image prune -a                  # remove ALL unused images (careful!)
docker image prune -a --filter "until=168h"   # unused images older than 7 days

Windows PowerShell note: Set BuildKit inline instead of prefixing the variable:

$env:DOCKER_BUILDKIT=1; docker build -t my-app .

4. Docker Compose

# Start/stop
docker compose up -d                   # start all services detached
docker compose up -d --build           # rebuild images before starting
docker compose down                    # stop and remove containers
docker compose down -v                 # also remove volumes (DESTROYS DATA)

# Monitoring
docker compose ps                      # list services
docker compose logs -f api             # follow logs for specific service
docker compose logs --tail 50          # last 50 lines all services

# Interaction
docker compose exec api /bin/sh        # shell into running service
docker compose run --rm api npm test   # one-off command (new container)
docker compose restart api             # restart specific service

# Validation
docker compose config                  # validate and view resolved config

Minimal compose.yml example:

services:
  api:
    build: .
    ports:
      - "3000:3000"
    environment:
      - DATABASE_URL=postgres://user:pass@db:5432/mydb
    depends_on:
      db:
        condition: service_healthy

  db:
    image: postgres:16-alpine
    environment:
      POSTGRES_USER: user
      POSTGRES_PASSWORD: pass
      POSTGRES_DB: mydb
    volumes:
      - pgdata:/var/lib/postgresql/data
    healthcheck:
      test: ["CMD-SHELL", "pg_isready -U user"]
      interval: 10s
      timeout: 5s
      retries: 5

volumes:
  pgdata:

5. Volumes and networks

# Volumes
docker volume ls                       # list volumes
docker volume create mydata            # create named volume
docker volume inspect mydata           # details (mount point, etc.)
docker volume rm mydata                # remove (fails if in use)
docker volume prune                    # remove unused volumes

# Networks
docker network ls                      # list networks
docker network create mynet            # create bridge network
docker network inspect mynet           # details (connected containers)
docker network connect mynet NAME      # attach container to network
docker network disconnect mynet NAME   # detach container
docker network rm mynet                # remove network
docker network prune                   # remove unused networks

6. Disk usage and cleanup

Always start with a diagnostic before cleaning:

# Check what's using space
docker system df                       # summary
docker system df -v                    # detailed breakdown

# Targeted cleanup (safe)
docker container prune                 # stopped containers
docker image prune                     # dangling images
docker volume prune                    # unused volumes
docker network prune                   # unused networks

# Aggressive cleanup (confirm with user first!)
docker system prune                    # containers + images + networks
docker system prune -a                 # also unused images
docker system prune -a --volumes       # EVERYTHING — named volumes too

HARD RULE: Never run docker system prune -a --volumes without explicit confirmation from the user. This removes named volumes with potentially important data. Always ask first and explain what will be lost.

7. Dockerfile optimization

When reviewing or creating a Dockerfile, suggest these improvements:

1. Multi-stage builds — separate build environment from runtime to reduce final image size
2. Layer ordering — put dependencies before source code so changes don't invalidate cached layers
3. Combine RUN commands — fewer layers, smaller image
4. Use .dockerignore — exclude node_modules, .git, __pycache__, etc.
5. Pin base image versions — node:20-alpine not node:latest
6. Run as non-root — add USER instruction for security
7. Use slim/alpine bases — python:3.12-slim not python:3.12

Quick Reference

Pitfalls

Verification

After any Docker operation, verify the result with these checkable commands:

1. Container started?

   docker ps
   

   Expected: container appears with status Up.

2. Logs clean?

   docker logs --tail 20 NAME
   

   Expected: no error/exception lines in recent output.

3. Port accessible?

   curl -s http://localhost:PORT
   

   Or check port mapping:

   docker port NAME
   

   Expected: HTTP response or mapped port listing.

4. Image built?

   docker images | grep TAG
   

   Expected: image tag appears in list.

5. Compose stack healthy?

   docker compose ps
   

   Expected: all services show status running or healthy.

6. Disk freed?

   docker system df
   

   Expected: compare reclaimable space before and after prune — values should decrease.