What is Docker? How it works, benefits, and uses

Key takeaways:

• Docker helps you build, package, and run applications using Docker containers.
• Docker containerization helps prevent the frustrating “it works on my machine” problem.
• Containerized applications are easier to move between compatible environments and deploy without wasting server resources.

The image can then be used to run containers on both the developer’s computer and a compatible server. This can happen when your computer and the server use different software versions or when the server lacks a library the application relies on. Docker helps address this by packaging the application with the components it needs to run.

With these components packaged together, developers can use the same configuration during development, testing, and deployment without having to set everything up again at each stage. Let’s look at how Docker works, what it can help you do, and when it makes sense to use it.

What is Docker?

Docker is an open-source platform that uses container technology to build, share, and run applications. It provides tools for managing containers, which give applications an isolated place to run.

A Docker container is created from a container image, a template that contains the application and its required components. Unlike a virtual machine, a container shares the host operating system’s kernel rather than running a separate guest operating system. This typically makes containers lighter and quicker to start.

This process is called containerization, and Docker is one of the most widely used platforms for implementing it. It lets developers package applications into portable units that can run on compatible systems, including local computers, servers, and cloud platforms.

How Docker works

So, what happens when a developer uses Docker to get a website ready for launch? To understand the process, let’s start with a common problem developers face when moving an application from their computer to a server.

Before Docker: Why application deployment was difficult

When an application doesn’t run as expected, developers have to find out why. They may check dependencies, review settings, or adjust software versions until the application works properly. Developers often refer to this as the “it works on my machine” problem.

The process takes time, especially when the same issues appear on different machines. Without a reliable way to package an application’s requirements, teams spend extra time troubleshooting before they can release their software.

How Docker solves this problem

Docker lets developers package a website’s code and required components into a container image. This gives the team a defined package to work with, rather than having to gather and configure each component separately.

Docker doesn’t prevent every deployment issue, though. The server still needs to support the image, and external services or application settings require their own configuration.

Example: Running the same website application in different environments

A developer gets a small business website ready for Docker by bringing its code and required components together in a Docker image. The image can then be used to run containers on both the developer’s computer and a compatible server. Both containers use the same image, even though they run on different machines.

How Docker builds and runs containers

Docker gives developers a few simple ways to prepare an application and get it running in a container. They can follow three basic steps:

1. Describe what the application needs: The developer creates a file called a Dockerfile with instructions for setting up the application.
2. Create or download an image: The docker build command creates an image using those instructions. Developers can also use docker pull to download an existing image from an online registry.
3. Run the application: The docker run command uses the image to create and start a container.

Docker can also run several containers at once. For example, a website can have one container for the part visitors see and another for the code that handles requests behind the scenes. Both containers work together to run the website.

Components of Docker

Docker has several tools, each with a specific job in building and running applications. Here’s what each one does and how it fits into the process:

• Docker Engine: The core software that builds and runs containers.
• Docker client and daemon: The client lets developers send Docker commands, while the daemon handles tasks such as building images and running containers.
• Dockerfile: A text file that tells Docker how to build an image.
• Docker image and container: An image is a template for an application and its required components. A container is a running instance created from that image.
• Docker Hub and registries: Places where developers can find, store, and share Docker images.
• Docker Compose: A tool for defining and running applications that use multiple containers, such as a website with a separate backend and database.
• Docker Desktop: An application that helps developers build and run containers on their computers.

Each tool serves a different purpose, but they all work as part of the Docker platform. The tools you use depend on your project, the application you’re building, and how you plan to run it.

Benefits of Docker

Docker has several practical benefits for developers, especially when an application needs to run on different machines. Here are four benefits worth knowing before deciding whether it fits your project:

Consistent application environments

When developers test a new feature, they need to know whether a problem comes from the code or the setup it runs in. Docker helps narrow down that uncertainty by giving teams a defined set of application components to work with. This makes test results easier to interpret and helps developers spot issues related to their changes, rather than differences in the software setup.

Portability across different environments

Docker gives teams more flexibility when they decide where to run an application. A business can start with a server in its own data center and later move the application to a cloud platform, provided the new setup supports it. This can be useful as a project grows or hosting needs change. However, moving a container doesn’t automatically move everything the application depends on. Hardware, operating system requirements, settings, and external services still need to match the new setup.

Efficient resource utilization

Server resources are limited, especially when several applications need to run on the same machine. Docker containers share the host operating system’s kernel rather than each running a separate guest operating system, so they typically use less memory and storage than virtual machines. This gives teams room to run more application components on the same server hardware. The actual savings depend on the workload and server capacity, so Docker alone doesn’t guarantee a lower hosting bill.

Faster deployment and easier scaling

When a business releases an update or sees a rise in website traffic, its hosting setup needs to keep up. Docker supports automated build and release workflows that help teams test changes before they go live. When an application needs to handle more requests, teams can also run additional container instances. Tools such as Kubernetes help manage these instances across multiple servers, although the results still depend on the available infrastructure and how the application is configured.

Common use cases of Docker

Docker has a place in several stages of software development. Developers use it to test changes before release, run applications on servers, and manage software made up of several connected parts. Here are four common use cases and how they work in practice.

Application development and testing

Before releasing a website or application, developers need to check that their changes work as expected. Docker Desktop lets them run containers on their own computers with the software versions and dependencies the application needs. For example, a team building a business website can test a new feature inside a container before making it available to visitors. Developers can also recreate the same setup when a bug appears, which helps them investigate the problem without rebuilding the environment each time.

Web application deployment

Once an application is ready, the team needs a way to run it on a server. Docker images provide a consistent starting point for deploying applications to compatible servers or cloud platforms. An online store, for example, can run its frontend in one container and its backend in another. Each component has its own environment, but both work together to serve the website. The team still needs to configure hosting, networking, storage, and security, as well as any external services the application relies on.

Microservices and multi-container applications

Separate containers become especially useful when an application has several parts that need to work together. With a microservices architecture, developers split an application into smaller services that they can build and manage independently. A business website, for instance, can have separate containers for its frontend, backend, and database. Docker Compose helps developers define and run these related services as one application. For larger setups that span multiple servers, container orchestration tools such as Kubernetes or Docker Swarm help teams manage container deployment, scaling, and recovery.

Continuous integration and continuous deployment (CI/CD)

Docker also fits into automated workflows that help teams test and release software updates. With continuous integration and continuous deployment (CI/CD), a pipeline can run tests whenever a developer changes the code, then use docker build to create a new Docker image. The workflow can store that image in a container registry until it passes the required checks and is ready for deployment. This gives the team a repeatable process for preparing each release, although a successful deployment still depends on the quality of the tests and the pipeline’s configuration.

Docker vs. virtual machines vs. Kubernetes

Docker, virtual machines, and Kubernetes all play a role in running applications, but they don’t do the same job. Knowing what each one does helps you understand which tools your project needs and when they work well together.

Docker containers vs. virtual machines

The biggest difference is how each technology handles the operating system. Docker containers share the host operating system’s kernel, while virtual machines (VMs) run their own guest operating systems on virtualized hardware. Because containers don’t need a separate operating system for each instance, they typically use fewer resources and start faster. VMs provide a separate operating system environment, which is useful when an application needs a different OS or stronger separation at the operating system level.

Neither option is right for every situation. Your choice depends on the application’s requirements, the level of isolation you need, and the server hardware available.

Docker vs. Kubernetes

Docker and Kubernetes solve different problems, so teams often use them together. Docker provides tools to build, share, and run containers, while Kubernetes is a container orchestration platform that manages containerized applications across multiple machines. It helps teams deploy applications, scale workloads, and recover when containers fail.

For example, a developer can use Docker to build an image and test an application locally. When the application needs to run across several servers, Kubernetes manages its containers as demand changes. It requires a compatible container runtime but does not require Docker Engine specifically.

As an open-source project supported by the Cloud Native Computing Foundation, Kubernetes is widely used to manage applications in cloud native environments. Docker helps prepare and run containers, while Kubernetes coordinates them across a larger setup.

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Frequently asked questions

Build with confidence using the right tools

Docker is a useful tool for developers, but you don’t need to learn container technology to build a website for your business. Think about what your site needs, then choose a setup you’re comfortable managing.

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