What is Docker?

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What is Docker?

Docker is a platform designed to simplify the process of developing, deploying, and running applications by using containers. Containers allow you to package an application with all of its dependencies into a single, standardized unit. This encapsulation ensures that the application works seamlessly in any environment, from development to testing to production. Docker abstracts away the complexity of the underlying systems, making it easier for developers to create ready-to-run containerized applications.

Docker’s approach to containerization is both efficient and lightweight, allowing applications to be isolated in containers but share the same operating system kernel. This architecture minimizes overhead and maximizes performance compared to traditional virtual machines. With Docker, developers can focus on writing code without worrying about the environment where the application will ultimately run.

How containers work

Containers are the heart of Docker. Each container is an isolated environment, containing everything the application needs to run: code, runtime, system tools, and libraries. Unlike virtual machines, containers do not bundle a full operating system but instead share the host system’s kernel, making them more efficient, faster to start, and less resource-intensive.

Key features of containers include:

  • Isolation: Containers separate application workloads, ensuring that processes do not interfere with each other.

  • Portability: Applications within containers can be moved easily across different environments.

  • Efficiency: Containers use system resources more effectively than virtual machines.

To create Docker containers, developers first define a template, called an image. With this template, Docker can create an unlimited number of identical containers, making it easy to scale up a containerized application or service to meet fluctuations in demand.

The ability to quickly and reliably create multiple instances of an application is one of the reasons Docker is so powerful for deployment and scalability. It simplifies the process of managing and distributing software, regardless of the underlying infrastructure.

Benefits of Docker

Docker introduces a paradigm shift in how developers build and deploy applications, offering significant advantages over traditional methods. Most notably, Docker makes application development and deployment more stable, efficient, and repeatable:

  • Stable environments: Docker containers ensure that applications run in the same environment regardless of where they are deployed. This consistency ensures that the application behaves the same in development, testing, and production, minimizing environment-related issues and reducing debugging time.

  • Efficient resource utilization: Unlike virtual machines that require a full-blown operating system, Docker containers share the host’s OS, resulting in a significantly lighter footprint. This efficient use of system resources means that you can run multiple containers on a single host, maximizing resource utilization and reducing costs.

  • Repeatable deployment processes: Docker containers are created from images that can be versioned and stored in a registry. This means that every time you deploy a container, you’re using an identical environment setup. This repeatability ensures that the deployment process is straightforward, less prone to errors, and easily automated, enhancing the reliability of deployments.

Teams that build applications in Docker experience a range of benefits over traditional application development methods. These benefits include:

  • Accelerated development cycles: With Docker, developers can spin up containers in seconds. This rapid turnaround time facilitates continuous integration and continuous delivery (CI/CD) practices, enabling teams to iterate on and deliver features and fixes more quickly.

  • Improved collaboration: Since Docker containers are consistent across all environments, developers, QA testers, and operations teams can work in sync, a core principle of Agile development and DevOps. Everyone works with the same container setup, which reduces discrepancies and misunderstandings.

  • Reduced infrastructure costs: By optimizing resource utilization and allowing multiple containers to run on a single host, Docker reduces the need for numerous servers or virtual machines. This consolidation can lead to substantial cost savings in terms of hardware and maintenance.

Docker’s approach to containerization aligns with business goals by enabling faster delivery, better team collaboration, enhanced security, and cost-efficiency.

Docker terms and technologies

To effectively use Docker, familiarity with its key terminologies and associated tools is essential.

Docker key terms

A text document that contains all the commands a user could call on the command line to assemble an image. It's the blueprint for creating Docker images.

Docker images

Read-only templates used to create containers. Images include the application code, libraries, tools, dependencies, and other files needed to run the application.

Docker containers

Runnable instances of Docker images that encapsulate the application and its environment, providing an isolated environment for the application to run.

Docker Engine

The core of Docker, it's the runtime that builds and runs Docker containers, allowing users to build, distribute, and run Docker containers on any compatible platform.

Docker Hub

A cloud service for sharing applications packaged as Docker images. It's a centralized resource for container image discovery, distribution, and change management, facilitating collaboration and sharing within the Docker community.

Understanding these terms and how they interrelate is crucial for leveraging Docker’s full potential in application development and deployment.

Similarly, familiarizing oneself with Docker’s ecosystem of tools can significantly enhance the development, deployment, and management of containerized applications.

Docker tools
Docker Engine

The runtime that builds and runs Docker containers, allowing users to build, distribute, and run Docker containers on any compatible platform

Docker Hub

A cloud service for sharing applications packaged as Docker images, facilitating collaboration within the Docker community

Docker Compose

A tool for defining and running multi-container Docker applications.

Docker Swarm

Docker's native container orchestration tool for better management of containers across different nodes

Docker CLI

Allows users to interact with Docker directly from the command line, providing a direct way to control and automate Docker tasks

Docker Desktop

An application for Mac and Windows machines that facilitates building and sharing containerized applications and services


An open-source platform for automating deployment, scaling, and operations of application containers across clusters of hosts. While not a Docker-native tool, Kubernetes integrates deeply with Docker to provide a powerful orchestration solution.

Docker’s suite of tools and technologies are an essential part of the modern development toolkit. Understanding the Docker ecosystem gives teams flexibility and efficiency in building, deploying, and managing containerized applications.

Docker and CI/CD

Docker plays an important role in CI/CD pipelines, enhancing the automation, consistency, and reliability of application builds and deployments.

Here are a few ways Docker supports effective CI/CD practices:

  • Seamless integration: In CI, developers frequently merge code changes into a shared repository. Docker aids this by ensuring that code runs uniformly across environments, reducing integration challenges that arise due to discrepancies in operating environments.

  • Efficient testing: Docker enhances CI by facilitating automated testing. Containers can be quickly spun up with specific configurations required for running tests. Testing environments can be created and disposed of with minimal overhead, speeding up the testing cycle.

  • Reliable deployment: For CD, Docker containers ensure that the application environment remains unchanged throughout the deployment pipeline. This consistency minimizes the risk of deployment failures due to environment-specific issues.

  • Scalability and rollback: Docker simplifies scaling applications in response to demand and enables easy rollback to previous versions if needed. This flexibility is crucial for maintaining high availability and quickly addressing any issues that arise post-deployment.

  • Resource optimization: Docker’s efficient use of system resources is vital for CI/CD pipelines, as it allows for parallel execution of tasks such as testing different features of an application without requiring extensive hardware resources.

  • Integration with CI/CD tools: Docker easily integrates with popular CI/CD tools. This compatibility allows you to automate the entire pipeline within Docker environments.

Docker’s containerization technology is the perfect complement to CI/CD pipelines, addressing key challenges in application development and deployment. By enabling faster, more reliable workflows, Docker enhances the efficiency of CI/CD, allowing teams to deploy high-quality software at a faster pace.

Build on Docker with CircleCI

To get the most out of your Docker workflows, you need effective automation. CircleCI’s industry-leading CI/CD platform is purpose-built to help you capture the benefits of containerization throughout your development cycle.

Key features of CircleCI’s docker support include:

  • Docker executor: Run CI/CD jobs in a Docker container using our fleet of optimized convenience images or your own custom images.

  • Docker layer caching: Reduce build times by reusing the data from previously built Docker images.

  • Unlimited parallelism: Make the most of your build minutes by running multiple Docker jobs at once.

  • Rapid debugging: SSH directly into your running Docker containers to debug failed builds in real time, keeping your team focused and on track.

  • Easy integrations: Connect with your favorite registries or cloud platforms, scan your containers for vulnerabilities or performance issues, or deploy to your preferred container orchestration in a single line of code.

Learn more about CircleCI’s first-class Docker support

Join the ranks of leading companies and development teams who trust CircleCI for their Docker-based CI/CD needs. Sign up for a free account to get 6,000 monthly build minutes, access to our lightning-fast convenience images, and a host of additional features that will unlock the true potential of containerization in your organization.