Essential Docker Concepts and Workflows

Aug 9, 2024

Docker Crash Course Summary

Introduction to Docker

  • Goal: Teach main concepts of Docker and provide hands-on experience.
  • Purpose: Improve engineering skills and understanding of Docker for software development and deployment.

What is Docker?

  • Definition: Docker is a virtualization software that packages applications into containers.
  • Purpose: Simplifies the development and deployment of applications by including application code, libraries, dependencies, runtime, and environment configuration in a single package.
  • Improvement: Docker improves over traditional virtual machines by standardizing the process and reducing setup complexity.

Why Use Docker?

  • Problems Before Docker:
    • Developers had to install dependencies and services directly on their OS, leading to inconsistencies and configuration errors.
    • Installation processes differed across operating systems, increasing complexity.
  • Solutions with Docker:
    • Services packaged in isolated containers reduce installation overhead.
    • Developers can focus on code rather than installations and configurations.
    • Different versions of applications can run simultaneously without conflict.

Docker vs. Virtual Machines

  • Key Differences:
    • Docker virtualizes the application layer and uses the host OS kernel, resulting in smaller image sizes and faster startup times.
    • Virtual machines have their own kernel and OS, leading to larger images and longer startup.
    • Compatibility: Docker is limited to the host OS kernel, while VMs can run different OS types.
  • Docker Desktop: Enables the use of Docker on non-Linux systems like Windows and macOS by providing a Linux kernel via a hypervisor.

Installing Docker

  • Installation Steps: Refer to the official Docker installation guide for up-to-date instructions depending on the operating system.
  • Components of Docker Desktop:
    • Docker Engine: Core service for virtualization.
    • Command-line interface: For executing Docker commands.
    • Graphical user interface: Alternative for users preferring UI over command-line.

Key Docker Concepts

Docker Images

  • Definition: A Docker image is a read-only template that contains application code, dependencies, and environment configuration.
  • Creation: Images can be created from a Dockerfile, which defines the structure and instructions for building images.

Docker Containers

  • Definition: A container is a running instance of a Docker image.
  • Creation: Multiple containers can be created from a single image, facilitating scalability.

Docker Registries

  • Public Registry: Docker Hub is the largest public registry for Docker images.
  • Private Registry: Companies can create private registries to store proprietary images.
  • Repositories: Each application can have its own repository within a registry for storing different versions.

Creating Custom Docker Images

  • Dockerfile: A text file with instructions on how to build a Docker image.
    • Key Directives:
      • FROM: Specify the base image.
      • COPY: Copy files into the container.
      • RUN: Execute commands during the image build.
      • CMD: Define the command to run when the container starts.
  • Building an Image: Use docker build command to create an image from a Dockerfile.

Running Docker Containers

  • Running a Container: Use docker run command with specified image and ports for accessibility.
  • Port Binding: Expose container ports to the local host to access applications running inside containers.

Docker in Software Development Lifecycle

  • Workflow:
    1. Local development with Docker containers for databases/services.
    2. Use CI/CD pipelines to build Docker images from application artifacts.
    3. Push Docker images to private repositories.
    4. Pull images to development or production environments for testing/deployment.

Conclusion

  • Understanding of basic Docker concepts: images, containers, registries, and how Docker simplifies development workflows.
  • Further learning recommended for advanced topics like Docker Compose, Docker Volumes, and integration with CI/CD processes.