Continuous integration and continuous delivery (CI/CD) are key practices for delivering software quickly and reliably. Docker is a great tool to enable CI/CD workflows. In this beginner’s guide, we will cover how to use Docker for CI/CD.

What is Continuous Integration and Delivery?

Continuous integration (CI) and continuous delivery (CD) are related practices that allow teams to deliver software changes more frequently and reliably.

CI means merging developer code changes frequently into a shared repository like Git. Each merge is then verified by an automated build and test process. This catches issues early and keeps the master branch stable.

CD takes the CI process further by deploying each code change that passes tests into production-like environments. CD makes releases low risk events that can be performed anytime.

Used together, CI and CD accelerate the software development and release cycle. Teams can ship updates in hours or days, rather than weeks or months. This helps meet business needs faster.

Benefits of Using Docker for CI/CD

Docker is a perfect match for CI/CD pipelines. Here are some key benefits:

• Consistent environments – Docker guarantees code is tested in the same environment every time. No more “works on my machine” bugs!
• Faster setup – Docker containers spin up instantly. There’s no need to manually configure and tear down CI infrastructure.
• Isolation – Docker sandboxing prevents test failures due to environment conflicts. Tests can run in parallel safely.
• Portability – Docker images built during CI can be deployed intact to any environment. No surprises from machine-specific differences.
• Scalability – Docker lets you scale CI pipelines elastically by firing up more containers on demand.

By leveraging Docker to containerize the CI/CD toolchain, teams can build, test, and deploy applications faster and with more confidence.

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Docker Components for CI/CD

There are two main Docker components used to enable CI/CD workflows:

Docker Engine

The Docker Engine runs on development machines, CI/CD servers, and production servers. It is used to build, run, and distribute Docker containers.

Key features like images, containers, and Docker Compose make the engine ideal for CI/CD:

• Build reproducible application images during CI
• Use containers to isolate development, staging, and production
• Leverage Docker Compose to define CI services and dependencies

Docker Registry

A Docker registry stores and distributes Docker images. Docker Hub is the default public registry.

Registries are useful for CI/CD because:

• Built images can be easily shared between developers
• CI/CD pipelines can pull prebuilt images to start jobs
• Finished application images can be stored for later deployment

Using a registry avoids having to transfer Docker images manually at each stage.

CI/CD Pipeline with Docker

Now let’s walk through a sample CI/CD pipeline powered by Docker. We will look at the steps to continuously build, test, and deploy a web application.

1. Developer commits code changes

The pipeline starts when a developer makes a Git commit to the central code repository. This triggers the continuous integration process.

The commit consists of changes to the application source code and Dockerfiles used to build images.

2. CI server builds new Docker images

The CI server detects the new Git commit and starts a new build job.

It uses Dockerfiles to build Docker images containing the application and its dependencies. These immutable images encapsulate the new changes.

Automated unit, integration, and functional tests can also run at this build stage to verify application changes.

3. Docker registry stores images

Once successfully built and tested, the Docker images are pushed to the registry. This caches them for use in future pipeline stages.

Developers can pull these images to verify fixes or run integration tests. The images can also be used for demos or bug reproduction.

4. CD system deploys images

With new images available, the continuous delivery system can deploy them through staging and production environments.

Docker Swarm or Kubernetes is typically used to manage container deployments across multiple servers. Containers make CD predictable and low risk.

5. Monitoring checks health

Finally, monitoring tools like Prometheus check the updated application’s health. Alerts notify the team if any performance or usage anomalies are detected.

The cycle then continues with the next code change!

Continuous Integration Tools for Docker CI/CD

Many excellent open-source tools exist for building Docker-powered CI/CD pipelines. Here are some top options:

Jenkins

Jenkins is the most popular self-hosted CI/CD automation server. The Docker Pipeline and Docker plugin make it easy to use Docker for builds and tests.

CircleCI

CircleCI is a hosted CI/CD service that natively supports Docker. Build Dockerfiles, run containers for isolation, tag images, and push to registries seamlessly.

GitHub Actions

GitHub Actions is an automated CI/CD workflow baked right into GitHub. Create workflows that build Docker images and deploy to Kubernetes with absolute minimum configuration.

Travis CI

Travis CI is another hosted CI solution with first class Docker support. Define .travis.yml files that build images, launch services for integration testing, and push to registries or cloud hosts.

Codeship

Codeship is a docker-native CI/CD platform designed to build, test, and deploy Docker-based applications with speed and consistency. Supports all major code repositories and cloud providers.

Best Practices for Docker CI/CD

Here are some key best practices to follow when implementing Docker for CI/CD:

• Use small base images – Keep images slim by starting from small base images like Alpine or Debian instead of heavy ones like Ubuntu.
• Build for one app per container – Build only your application in its Docker image, not a complex runtime like LAMP stack.
• Follow linter rules – Enable Dockerfile linters to enforce best practices like avoiding latest tags, small number of layers, and no root user.
• Leverage multistage builds – Use multistage builds to keep final images lean. Install tools needed for building in earlier stages.
• Minimize build contexts – When building images, optimize Docker build context to exclude unnecessary files and speed up Dockerfile instruction caching.
• Tag images intelligently – Use Git SHA or version tags rather than latest so it’s clear what code is in an image.

Following these patterns will ensure you get the most out of using Docker for CI and CD.

Conclusion

Docker opens up many possibilities for implementing modern CI/CD pipelines. Key Docker features like images, containers, and registries make it straightforward to build, test, store, and deploy applications reproducibly and at scale.

When combined with CI/CD tools like Jenkins, CircleCI, and Kubernetes, Docker enables rapid, reliable software delivery. Teams can ship updates faster while maintaining quality and stability.

In this guide, we looked at Docker concepts and components for CI/CD. We walked through a sample Docker CI/CD workflow. We also covered top tools and best practices.

Using Docker for continuous integration and delivery provides huge benefits for developer productivity, software quality, and business agility. By following these patterns, you can accelerate your team’s delivery of high-value software updates.