Docker - robbiehume/CS-Notes GitHub Wiki

• Best docker courses (free and paid)

• Docker is a tool that allows developers, sys-admins etc. to easily deploy their applications in a sandbox (called containers) to run on the host operating system i.e. Linux
• The key benefit of Docker is that it allows users to package an application with all of its dependencies into a standardized unit for software development
• Unlike virtual machines, containers do not have high overhead and hence enable more efficient usage of the underlying system and resources

• Portability – A containerized application runs the same way regardless of where it's deployed (local machine, cloud, server)
• Isolation – Containers run independently from one another and from the host OS, reducing conflicts between dependencies
• Lightweight – Unlike virtual machines (VMs), containers share the host OS kernel, making them much faster and more resource-efficient
• Scalability – Works well with microservices architectures, allowing easy scaling of individual services

Common Use Cases

• Microservices: Run each service in a separate container
• Development Environments: Developers can work with the same stack across machines
• CI/CD Pipelines: Automate testing and deployment
• Cloud Deployments: Easily run applications in cloud environments

Concept	Description
Container	A lightweight, isolated environment for running applications
Image	A snapshot of a container; contains the app and all dependencies
Dockerfile	A script defining how an image is built (OS, libraries, commands, etc.)
Registry	A storage for images (e.g., Docker Hub, AWS ECR, GitHub Container Registry)
Volumes	Persistent storage for data shared between containers
Network	Allows communication between containers

1. Write a Dockerfile
   • Defines the environment for your application
2. Build an Image
   • docker build -t myapp . → Converts the Dockerfile into an image
3. Run a Container from the Image
   • docker run -d -p 8080:80 myapp → Starts a container from the image, exposing it on port 8080
4. Share the Image
   • docker push myapp → Upload to a registry so others can use it
5. Deploy Anywhere
   • Run the container on any system with Docker installed

Feature	Docker (Containers)	Virtual Machines
Size	Small (MBs)	Large (GBs)
Startup Time	Seconds	Minutes
Performance	Near-native speed	Slower
Isolation	Shares OS Kernel	Full OS Isolation
Portability	Highly portable	Less portable

• An image is a package/template used to create one or more containers
• Containers are running instances of images that are isolated and have their own environments and set of processes

• VMs are great at providing full process isolation for applications: there are very few ways a problem in the host operating system can affect the software running in the guest operating system, and vice-versa
  • But this isolation comes at great cost — the computational overhead spent virtualizing hardware for a guest OS to use is substantial
• Containers take a different approach: by leveraging the low-level mechanics of the host operating system, containers provide most of the isolation of virtual machines at a fraction of the computing power
• There are already pre-built containers for common applications

• # Check Docker is installed
   docker --version
  # Pull an image from Docker Hub
    # Or it will automatically pull it when you do the run command for the first time
   docker pull nginx
  # List available images
   docker images
  # Creates a new container from an image and starts it in the background 
    # To run in foreground (attached), remove -d
   docker run -d -p 8080:80 nginx
  # List running containers
   docker ps
  # Stop a container
   docker stop <container_id>
  # Starts an existing stopped container
   docker start 
  # Remove a container
   docker rm <container_id>
  # Remove an image
   docker rmi <image_id>
  # View container logs
   docker logs <container_id>

• Stop all running containers: docker stop $(docker ps -aq)
• Remove all containers: docker rm $(docker ps -aq)
• Remove all images: docker rmi $(docker images -aq)

• When providing an ID to a command, you just need the first n characters that make it unique compared to others

• Option	Description
  -d	Runs the container in detached mode (background)
  -it	Interactive mode with a TTY (for CLI-based apps)
  --rm	Removes the container after it stops
  --name <name>	Assigns a custom name to the container
  --restart=<policy>	Defines the restart policy (always, unless-stopped, on-failure)