What is a Docker Registry?

If you deploy applications inside containers, there’s a good chance that you also use a Docker registry. Docker registries provide a simple, secure, and scalable way for developers to distribute containerized applications, and for users to search for and download them.

Keep reading for a deep dive into everything you need to know about modern Docker registries, including how they work, which options are available, how to set one up, and how to get started using it.

What Is Docker Registry?

The term “Docker registry” is a bit ambiguous because it can mean two different things.

In a narrow sense, Docker Registry (with a capital R) is an official tool from the Docker project for storing and distributing container images. Docker Registry is open source, and anyone can download and run the software to set up their own container image registry.

In a broader sense, it is any tool or service that can host and distribute container images. The official Docker Registry is one example of this type of solution, but there are many others, such as:

• Harbor, another open source option.
• Docker Hub, the official cloud-based registry from Docker. Unlike the open source Docker Registry, Docker Hub is a fully managed service that is available only from Docker.
• JFrog Artifactory, a binary manager that can be used to host container images both on-premises and in the cloud.
• Amazon Elastic Container Registry, a Docker registry service in the AWS cloud.
• Azure Container Registry, the main hosted container registry solution on Azure.
• Google Cloud Container Registry.

This article focuses on the official Docker Registry, but many of the points apply to any type of registry solution.

The Benefits of Docker Registry

The main benefits of using Docker Registry include:

• A centralized location for storing container images: Centralized storage helps businesses to keep track of the container images they use in an efficient manner, without having to manage multiple images spread across different repositories.
• A central location where users can find container images: Whether your users are employees of your business who need to access apps that you develop internally, external customers, or both, Docker Registry provides an easy-to-access place where they can search for and download container images.
• Access controls: Most registries provide access control features that allow teams to manage who can access which container images. For example, in most cases, you can allow some images to be downloaded by the public at large while making others available only to certain authenticated users.
• Image versioning: Docker Registry allows users to specify a version when downloading images and running them with Docker, Kubernetes, or a similar tool. Version management is useful in situations where some users need a particular version of an app (which may not be the latest version available), or when developers want to do things like make beta or alpha versions of their apps available while simultaneously offering stable releases.
• Docker and Kubernetes integration: Docker Registry is designed to integrate natively with Docker’s other tools and with Kubernetes, such that you can download and run container images from a registry using a single command. This integration saves users from having to download images, then upload them into a Docker or Kubernetes environment in order to use them. Most of the process is automated once you run a container to start a container based on a particular image.

Strictly speaking, Docker Registry isn’t the only way to host or share container images. You could store container images using a network file share. But that approach would make it more difficult for users to find and search for container images. It also wouldn’t support features like the ability to request specific versions of a container image when running it inside Docker or Kubernetes.

Another option is to host container images on a source code repository platform like GitHub, which provides many of the access control and versioning features you can get from a Docker registry. However, GitHub and similar platforms are designed primarily to host source code, not container images or other application binaries, and they lack the Docker and Kubernetes integrations that container registries provide. (GitHub Packages, a Docker registry service available from GitHub, is designed to host container images, but it’s separate from GitHub’s primary service, which is source code repositories.)

So, Docker Registry and other Docker container hosting services provide unique benefits that you wouldn’t get if you relied on other types of tools to manage or share container images.

How Does It Work?

To understand how Docker registries work, first consider that there are two main groups of Docker Registry users.

The first is developers who create containerized applications and want to share their images. These developers typically set up Docker Registry by deploying it as a container image. Then, they create repositories (which are essentially virtual directories that can host different container images) within their Docker Registry instance. From there, they push images for the applications they build into the registry and define the access policies that govern who can access those images.

The other group of Docker Registry users is application consumers, meaning people who want to run applications. For this group, Docker Registry works by providing a central platform where they can search for and download container images. Users typically need to configure their Docker or Kubernetes environment to integrate with their registry by specifying the registry’s location and port on the network when they download images.

How to Use Docker Registry

To contextualize the description of how Docker Registry works, here are a few examples of how you might interact with a Docker Registry instance over the command line:

To download a container image for Alpine Linux, use a command like:

docker pull alpineCode language: Perl (perl)

You could also specify a certain image version with:

docker pull alpine:3.14Code language: Perl (perl)

The docker pull commands above will download a container image to your local system, but they won’t start a container based on the image. If you want to download and run a container in a single step, use a command such as:

docker run alpineCode language: Perl (perl)

The docker run command automatically downloads the container image from a registry (unless a local copy already exists), then starts a container based on it.

To add a container image to a registry, first tag the image with the network address and port of your Docker registry along with the image name. For example:

docker image tag my-image:latest myregistryserver.com:1234/repo/my-image:latestCode language: Perl (perl)

Then, use the docker push command to add the image to the registry:

docker image push myregistryserver.com:1234/repo/my-image:latestCode language: Perl (perl)

This example shows how to use Docker’s native tooling to work with registries. In a production setting, you’d more likely integrate your CI/CD tools with your Docker registry in order to push images to the registry automatically whenever a new image becomes available.

How to Set Up a Docker Registry Server

To run a registry on your own server using the official Docker Registry tool, you can start it with a command as simple as:

docker run -d -p 1234:1235 --name registry-server registry:2Code language: Perl (perl)

This command pulls the official image for the open source Docker Registry from Docker Hub, then starts it as a container running on port 1234. Although running Docker Registry this way isn’t practical for production purposes because the registry server will shut down when the container stops, this approach is a handy means of experimenting with Docker Registry or deploying a registry quickly for testing purposes within a local development environment.

To host Docker Registry persistently, you can run it within a Kubernetes environment by creating a Pod and Service like the following:

apiVersion: v1

kind: Pod

metadata:

name: my-docker-registry-pod

labels:

app: registry

spec:

containers:

- name: registry

image: registry:2.8.1

---

apiVersion: v1

kind: Service

metadata:

name: docker-registry

spec:

selector:

app: registry

ports:

- port: 1234

targetPort: 1234Code language: Perl (perl)

This tells Kubernetes to run a Pod based on the Docker Registry container image version 2.8.1 and expose it on port 1234.

Docker Registry Security

While all mainstream Docker registry solutions aim to be secure by default, security-conscious users can take extra steps to harden their Docker registry environments. Consider the following practices:

• Unless you want to share your images with the public at large, define access controls to restrict who can view your Docker registry contents.
• If you’re hosting private images, run your registry on a non-standard port in order to make it harder for malicious users to discover.
• Place your registry behind a firewall, which is another way of protecting private registries against malicious access.
• Update your Docker registry software regularly to address security vulnerabilities.
• Remove unnecessary images from your registry. The fewer images you host, the easier it is to keep track of them and ensure that you don’t accidentally misconfigure access settings for sensitive applications.

Docker Registry Best Practices

In addition to the Docker registry security considerations we just described, the following best practices can help you get the most out of Docker registries in general:

• Choose the right type of registry for you: Hosted registries are the easiest type of Docker registry solution, but they also give you less control than on-premises or local registries.
• Specify image tags: Whenever you’re working with images inside a Docker registry, it’s a best practice to specify the tag of the image you are working with. If you don’t, the registry typically assumes you want the most recent version of the image, which is not always the case.
• Use logs: Most Docker registries provide logging and auditing features, which are useful for helping to secure images and investigate security incidents.
• Limit image size: The larger the images inside your registry, the longer they will take to download, and the more bandwidth the downloads will consume – which could become an issue because some registries restrict monthly bandwidth or charge you for exceeding bandwidth caps. So, the smaller your images, the better.
• Restrict registries to Docker images: Although you can sometimes insert other types of data into a Docker registry, this is a bad practice in most cases. You should store only container images inside your registries. If you want to host other types of information, like non-containerized application binaries, use a hosting solution designed for it.