Table of Contents
Overview of Container Registries
Container registries are services that store and distribute container images. They act as centralized libraries where you can push your own images and pull images created by others. Instead of passing image files around manually, developers and systems reference images by a registry address, an image name, and a tag, such as my-registry.example.com/app/backend:1.2.0.
A registry makes images available over the network through a standard HTTP API. Docker and other container tools speak this API to upload and download image layers. This is what allows you to build an image on one machine and run it easily on many other machines in development, testing, and production.
A container registry is not the same as a container runtime. Registries store and serve images. Runtimes like Docker Engine or containerd pull those images and run containers from them.
Registries, Repositories, Images, and Tags
It is useful to separate a few related terms that often get mixed together. A registry is the entire server or service, for example Docker Hub or a private Harbor instance. Inside a registry you have repositories. A repository groups different versions of a single image, such as all versions of my-api. Inside a repository, each version is identified by tags, such as latest, 1.0, or 1.0.3-alpine.
When you see something like registry.example.com/team/my-api:1.0.3, you can read it as:
Registry: registry.example.com
Repository path: team/my-api
Tag: 1.0.3
The actual contents of an image are stored as layers and are also identified by a digest, usually a long sha256:... string. The digest is a cryptographic hash of the image content. While tags are convenient names that you can move to point to newer versions, the digest stays bound to a specific immutable image.
Tags are mutable references. A tag like latest can point to different image content over time. Digests are immutable. If you want to guarantee you always get the exact same image, pin by digest, not only by tag.
Public vs Private Registries
Container registries are available as public services or as private deployments. A public registry is open for anyone to pull from, although pushing usually requires an account. Docker Hub is the most familiar example. Public registries are convenient for open source images and for sharing base images across the whole community.
Private registries are used for proprietary or internal images that must not be exposed to the public internet. Many organizations run a private registry inside their own network or use a managed private registry from a cloud provider. This provides better control over access, network traffic, and compliance with internal rules.
A common pattern in real projects is to mix both. Teams pull trusted base images like ubuntu or alpine from a public registry, then build application specific images on top of them and push the resulting images into a private registry. That way sensitive application code and configuration never leave the organization controlled registry, while the team still benefits from public ecosystems.
Examples of Popular Registries
Several container registry implementations and services exist, some provided by vendors and some open source. Docker Hub is the original public registry used by many beginners. It hosts a wide range of official images for languages, operating systems, and databases.
Cloud providers offer registries that integrate closely with their platforms. Amazon Elastic Container Registry connects to AWS identity, networking, and orchestration tools. Google Artifact Registry and Azure Container Registry fill the same role for their respective clouds. These services often provide features like role based access control, image scanning, and replication across regions.
There are also registry products you can run yourself. Harbor, GitLab Container Registry, and JFrog Artifactory provide private registry functionality combined with additional features like access policies, retention rules, and security scanning. They are usually deployed on premises or in your own cloud accounts, often integrated with internal authentication.
Naming and Addressing Images in Registries
An image reference includes several parts that tell Docker where to find and how to interpret an image. The general form is:
$$
[\text{registry}/][\text{namespace}/]\text{repository}[:\text{tag}]@\text{digest}
$$
Not all parts are always present. If you omit the registry, Docker assumes a default, commonly Docker Hub. If you omit the tag, Docker uses latest. The digest is optional but useful for pinning.
For example:
nginx:1.27 refers to repository nginx, tag 1.27, on the default registry.
mycorp-registry.internal/backend/api:2.1.0 uses a custom registry host and a nested repository path.
alpine@sha256:abcdef... pins a specific image by digest, regardless of what any tag might point at.
Do not rely blindly on default registry and latest tag in production workflows. Always use explicit registries and tags or digests to avoid unexpected changes when base images are updated.
Authentication and Access Control
When you interact with a private registry, you usually need to authenticate. Docker supports logging in to a registry using credentials such as a username and password, an access token, or a cloud specific mechanism like IAM roles. Once authenticated, you can pull and push images according to the permissions configured on the registry side.
Access control is typically enforced at repository or project level. You might give read only access to some teams, read and write access to others, and keep some repositories restricted to a small group. Many enterprise registries integrate with centralized identity systems like LDAP or cloud identity providers, so you can manage groups and roles in one place.
Security policies in registries can limit who is allowed to push to important repositories. Some organizations require automated systems to verify signatures or to enforce naming conventions before accepting a push. These controls help prevent accidental overwrites and the introduction of unapproved images.
Versioning, Retention, and Cleanup
Registries need to cope with many versions of many images. Each tag in a repository usually maps to a particular version of your application. Over time this can produce large storage usage if old images are kept forever. To address this, registries often provide retention policies that automatically delete older tags or images that are no longer needed.
You can design your tagging strategy to work well with such policies. For example, you can use tags that reflect release channels like dev, staging, and prod, along with build specific tags like 1.4.2 or build-20250207-1234. A registry policy can keep the most recent builds for each channel and discard older ones, or it can keep all production tags while aggressively cleaning up test builds.
Some registries also support garbage collection. When tags are deleted, the underlying layers may still exist if other tags reference them. Garbage collection finds unreferenced layers and frees their storage. For large deployments, a regular cleanup schedule becomes part of normal registry operations.
Security and Image Scanning in Registries
Modern registries often include features that help with image security. Image scanning is a common capability. The registry analyzes images to detect known vulnerabilities in system libraries or language specific packages. These scanners compare the software versions found in the image against vulnerability databases and report issues with severity levels.
Policies can then be defined to block images with critical or high severity vulnerabilities from moving into certain environments. Some registries allow you to mark specific vulnerabilities as accepted exceptions for a period of time if there is no immediate fix. This brings security closer to the point where images are produced and stored, instead of leaving it as a later manual step.
Registries may also support signing and verification. An image signer can produce a signature for a particular digest. Consumers can then verify that the image they pull has a valid signature from a trusted signer. This protects against tampering between the builder and the runtime. The signature is usually stored alongside the image metadata in the registry.
Never assume an image from any registry is safe by default. Use registry features like scanning, signatures, and access control, and combine them with your own policies for image sources and base images.
Registries in CI/CD and Deployment Pipelines
Container registries play a central role in automated pipelines. A typical workflow is to build images in a continuous integration system, then push them to a registry as the single source of truth for deployable artifacts. Environments like staging and production then pull from the registry, not from the build system directly.
This separation simplifies promotion. Instead of rebuilding the same source code for each environment, you build once, push to the registry, and then deploy the exact same image digest to every environment. When combined with tags, you can promote an image by retagging it in the registry, for example from app:1.5.0-rc to app:1.5.0 once it has passed tests.
In large organizations, the registry can also act as a gate between code changes and deployments. Only images that pass automated checks and satisfy security policies are published to a specific repository that production systems use. This shifts enforcement from scripts scattered across many servers into a single, observable service.
Federation, Mirroring, and Proxies
As usage scales, teams sometimes face challenges with registry availability, bandwidth, and latency. To mitigate these, some registries support mirroring and proxying. A mirror is a copy of part of another registry kept closer to your environment. A proxy cache pulls images on demand from an upstream registry and caches layers locally.
For example, if your build and runtime environments cannot access the public internet directly, you can configure an internal registry to proxy Docker Hub. When an image is requested, the proxy fetches it once from the internet and then serves subsequent requests from its local cache. This improves reliability and can meet security requirements that restrict direct external connections.
Federation can also mean synchronizing specific repositories across multiple registry instances in different geographic regions. This reduces latency for remote teams and allows for resilience if one region has an outage. The exact mechanisms differ between registry products, but the result is that users see a unified namespace like company/app even when the underlying storage is distributed.
Choosing and Using Container Registries Effectively
Selecting a registry and integrating it into your workflow involve both technical and organizational considerations. If you are just starting, a public service can be enough, especially for personal projects and learning. As soon as sensitive code or data is involved, a private registry becomes important. For teams already committed to a cloud provider, the managed registry service in that cloud can simplify authentication and network integration.
You will also want to align your registry usage with your image strategy. This includes a clear naming scheme for repositories, a predictable tagging convention, rules for who can push and who can retag, and expectations for how long images remain available. Documenting this early prevents confusion when multiple teams begin to depend on shared images.
Finally, keep in mind that registries themselves are critical infrastructure. The availability and performance of your registry affect your entire container based workflow. Monitoring, backups, security updates, and capacity planning for registries are not optional side tasks. Treat them with the same care you give to your source control and production databases.