Docker-Cheat-Sheet

Following summary was generated by ChatGPT based on a conversation I had with it.

1. Docker Images and Tags

Default Tag (latest):
If no tag is specified during a build, Docker assigns the latest tag by default.
Example: docker build -t myapp . creates myapp:latest.
Versioned Tags:
Use versioned tags (e.g., myapp:1.0) to track changes and ensure reproducibility.
Example: docker build -t myapp:1.0 ..
Best Practices:
Use semantic versioning (e.g., 1.0.0, 1.1.0).
Optionally maintain a latest tag for the most recent version.
Automate tagging in CI/CD pipelines using build numbers, commit hashes, or timestamps.
Cleaning Up Images:
Remove dangling (unused) images: docker image prune.
Remove all unused images: docker image prune -a.

2. Docker Containers

Lifecycle:
Containers are ephemeral; any data stored inside a container is lost when it is removed.
Use volumes or bind mounts to persist data beyond the container lifecycle.
Recreating Containers:
Containers must be removed before being recreated.
Use docker-compose down followed by docker-compose up to safely recreate containers.
Inspecting Containers:
Use docker ps to list running containers.
Use docker inspect <container_id> to view detailed container information.

3. Docker Volumes

Purpose:
Volumes persist data independently of the container lifecycle.
Ideal for databases, file storage, or any data that must survive container recreation.
Using Volumes:

Define volumes in docker-compose.yml:

volumes:
  my-volume:
services:
  my-service:
    volumes:
      - my-volume:/path/in/container

List volumes: docker volume ls.
Remove unused volumes: docker volume prune.
Bind Mounts:
Use bind mounts to map a host directory to a container path:
```
volumes:
  - ./data:/path/in/container
```

4. Docker Compose

Service Configuration:

Define services in docker-compose.yml with image, build, ports, and volumes:

services:
  my-service:
    image: myapp:1.0
    build:
      context: .
      dockerfile: Dockerfile
    ports:
      - "8080:8080"
    volumes:
      - my-volume:/data

Ports:
Use the ports directive to map container ports to host ports:
```
ports:
  - "8080:8080" # Host:Container
```
Avoid port conflicts by assigning unique host ports for each service.
Rebuilding Services:
Force a rebuild of images: docker-compose up --build.
Pull updated images: docker-compose pull.

5. Dockerfile

EXPOSE Directive:
Documents the ports the container listens on but does not map them to the host.
Example:
```
EXPOSE 8080
```
Best Practices:

Use multi-stage builds to reduce image size:

FROM mcr.microsoft.com/dotnet/sdk:9.0 AS build
RUN dotnet build
FROM mcr.microsoft.com/dotnet/aspnet:9.0 AS runtime
COPY --from=build /app /app

Keep the Dockerfile clean and modular.

6. Cleaning Up Docker Resources

Images:
Remove unused images: docker image prune.
Remove specific images: docker rmi <image_id>.
Containers:
Stop and remove containers: docker rm -f <container_id>.
Volumes:
Remove unused volumes: docker volume prune.
Networks:
Remove unused networks: docker network prune.

7. Debugging and Verification

Inspect Running Containers:
Use docker ps to list running containers.
Use docker logs <container_id> to view logs.
Verify Ports:
Check mapped ports in docker-compose.yml or with docker ps.
Avoid conflicts by assigning unique host ports.
Test Changes:
Add unique log messages or version numbers to verify the latest image is running.

8. Automating with CI/CD

Automate builds and tagging:

docker build -t myapp:$(git rev-parse --short HEAD) .

Push images to a registry (e.g., Docker Hub, Azure Container Registry):
```
docker push myapp:1.0
```