What is a container registry?
A container registry is a place to store container images for use in application development—especially cloud native development on microservices and containerized applications.
MCR, Docker, YAML
How does a container registry work?
Container images—standalone packages of software that can be used to quickly build and run containerized applications and their dependencies—form the basis of the cloud native ecosystem. Used with container engines like Docker, container images transform the way that software is developed and delivered. But without a way to organize and share container images, they wouldn’t be nearly so useful.
At a Glance
A container registry is essentially a central service or repository designed to store and manage container images. You can think of it like a digital warehouse or recipe book where different container images are cataloged, tagged, versioned, and made available for quick retrieval. Once images are in a registry, teams across an organization (or from outside, in the case of a public registry) can easily find, pull, and run these images within their own environments.
The Push-Pull Workflow
1. Building the Image: Developers or automated tools (through a CI/CD pipeline) use a container engine (for example, Docker) to bundle an application’s code and dependencies into a single image. The resulting image is stored locally on the developer’s machine or build server.
2. Tagging and Versioning: Before uploading (or “pushing”) the image to a registry, it’s tagged with a meaningful reference (e.g., my-app:2.0). This tag makes it easier to track specific versions of the software and ensure clarity over which version is being deployed or tested.
3. Pushing to the Registry: The image is then “pushed” to the container registry. During the push process, the registry breaks the image down into layers (representing file system changes) and stores them efficiently—often deduplicating layers that already exist in the registry to save space.
4. Storing and Cataloging: Once the image is in the registry, it’s organized under a repository (e.g., my-org/my-app) and can be pulled by authorized users or automated systems. Metadata, such as the image’s creation date, author, and vulnerabilities (if scanned), is also stored, helping teams track image details over time.
5. Pulling the Image: When developers or automated deployment tools need to run the application, they “pull” the image from the registry. The registry retrieves and assembles all the required layers and delivers them to the requesting environment—whether that’s a local workstation, a testing environment, or a production cluster.
Integration with CI/CD Pipelines
A key advantage of container registries is how seamlessly they integrate with Continuous Integration and Continuous Delivery (CI/CD) workflows. After each code commit or update:
Build Process: The CI tool automatically builds a new container image.
Testing & Scanning: This image is tested, scanned for vulnerabilities, and possibly subjected to policy checks (e.g., code quality or compliance requirements).
Publishing: Upon successful tests and checks, the pipeline automatically pushes the new, verified image to the container registry.
Deployment: A CD system retrieves (pulls) the newly published image from the registry to deploy it in the target environment.
This loop keeps development teams agile, ensuring fast feedback and consistent, reproducible environments at every stage.
Layered Storage and Caching
To optimize storage and speed, container registries store images in a layered format. When an image is built, each instruction in the Dockerfile (or equivalent) typically creates a new layer. For example, if one layer includes the operating system files and another adds a programming language runtime, these layers can be reused by future images. This reduces redundant data transfers, making the push/pull process faster and saving storage space in the registry.
Version Control and Image History
Another advantage is version control. Each image in a registry can be tagged with multiple tags (e.g., latest, stable, v1.2, etc.), and some registries keep a history of which layers changed when. This transparency:
Helps teams roll back to previous versions in case of issues.
Provides a clear record of where each image came from and which changes went into it.
Allows easy comparison of image “footprints” and resource usage over time.
Security and Access Control
Because container images can contain critical software and sensitive data, container registries often provide mechanisms for access control. This includes:
Authentication and Authorization: Users must log in with valid credentials, and only users with the right permissions can push or pull certain images.
Scanning and Policy Enforcement: Many registries integrate with security scanners to identify vulnerabilities or outdated libraries, preventing deployments of insecure images.
Role-Based Access Control (RBAC): Administrators can define different roles (e.g., read-only, developer, admin) and assign them to users or groups.
Without a registry, container images would remain on individual machines, limiting their value. By providing a central place to store and manage images, container registries enable standardized workflows, streamline sharing and collaboration, and enforce security policies. Simply put, the container registry is the linchpin that makes large-scale, cloud native development feasible—ensuring every team member (or system) can quickly and reliably access the exact version of every container image they need.
What is the difference between a container registry and a repository?
When people talk about container registries, they sometimes use the words “registry” and “repository” interchangeably. That can lead to some confusion on the terms, but the difference is really pretty simple.
A repository (or “repo” for short) is a general term for any specific digital location where software packages are stored. Often, the repository is subject to version control and there is metadata associated with it, so users can keep track of the package and any changes that are made to it. When we refer to the places where packages and version control records are stored on GitHub, for example, we call them “GitHub repos.”
A container registry, on the other hand, is a specific location for storing container images. In a sense, you can think of it as a type of repository—but crucially, it may include one or more discrete repositories for those images, in the same way that a library may include multiple separate collections.
What are the benefits of a container registry?
In cloud native application development, container images can act as modular components. If a developer wants to spin up a fresh instance of a Postgres database, she doesn’t have to provision new resources or spend a great deal of time configuring the database—she can simply download a Docker image of the database and its dependencies.
But the container registry provides more than building blocks. It can also…
Provide a place to save images for sharing and collaboration
Act as a single source of truth for an application or component, providing the most recent version ready for replication and use
Define container images approved for use in the organization
These are some of the most common uses of container image registries, but private registries can carry that functionality still further.
Container registry types
Public Container Registries
A public container registry is a freely accessible platform where anyone can upload and download container images. Examples include Docker Hub, GitHub Container Registry (GHCR), and Quay.io. Public registries are typically the first registries developers use because they come preconfigured with many container engines—for example, Docker defaults to Docker Hub.
Pros:
Easy to get started: no setup required and many popular images (e.g., Ubuntu, NGINX, Postgres) are readily available.
Massive ecosystem: a large community contributes and maintains images, which can help accelerate development.
Simplified sharing: open-source projects and smaller teams can conveniently publish and distribute images for free.
Cons:
Limited control: because it’s public, you have no direct control over the hosting environment or who can upload images.
Potential security risks: images may contain vulnerabilities or malicious code. A recent security study showed that many public registry images contain exploitable vulnerabilities, making it risky to trust them without additional scanning and validation.
Private Container Registries
A private container registry restricts access to a defined set of users—either within a single organization or a smaller group of trusted collaborators. These registries are typically hosted on-premises, in a private cloud, or as a managed service (e.g., Mirantis Secure Registry, AWS ECR, or Azure Container Registry) and require authentication or role-based access control (RBAC).
Pros:
Enhanced security: since private registries can limit who can push and pull images, you gain better control of security policies and reduce the likelihood of unauthorized image access.
Strong governance: role-based access, policy-driven validation, and mandatory image scanning are simpler to implement.
Official “single source of truth”: teams know exactly which images are approved for production and can track version histories more rigorously.
Cons:
Setup and maintenance: configuring, securing, and managing private registries often requires more resources, both in terms of infrastructure and administrative overhead.
Upfront costs: while many private registry solutions include advanced features, they may come with licensing or cloud hosting fees.
Public container registry vs. private container registry
Public registries are freely available online. Anyone can download container images from them, and–after signing up for an account—anyone can save images to them as well. Docker Hub is the most widely used public registry, and it is typically the registry a developer encounters first, since it is the default registry for Docker tools.
A public registry such as Docker Hub can be a good fit for individual developers who are learning the ropes, or for small open source projects. But because anyone can contribute to the Docker Hub registry and other public registries, images may be contaminated: a security firm reported that more than half of all images on Docker Hub included exploitable vulnerabilities.
For enterprises and other organizations running mission critical software, it’s essential to understand exactly what is in their container registries. This is why they use private registries. A private registry is a container registry that limits access to a defined set of users. It may be hosted on-prem or by a service provider, but either way it will typically include some form of role-based access control (RBAC) enabling the organization to specify who can contribute to the registry and how.
Securing a container registry
Because container images can contain everything from operating system packages and libraries to application code and dependencies, securing a container registry is critical for any organization relying on containers in production. Here are some best practices to consider:
Implement Role-Based Access Control (RBAC)
Ensure only authorized users can push, pull, or modify images.
Assign permissions based on user roles (e.g., administrators, developers, QA testers) to limit the risk of accidental or malicious changes.
Require Authentication and Encryption
Use secure protocols like HTTPS/TLS to encrypt data in transit.
Store credentials securely—either via a secrets management tool or using built-in registry capabilities.
Consider encrypting images at rest, especially if hosting the registry in an external environment.
Conduct Automated Vulnerability Scans
Regularly scan container images, ideally at each step in the CI/CD pipeline.
Use a vulnerability database that’s frequently updated to catch newly discovered threats.
Look for solutions that provide deep scanning at the binary level for maximum insight into hidden vulnerabilities.
Use Image Signing
Employ cryptographic signing (such as Notary or Cosign) to validate image authenticity.
Signed images help verify that no tampering has occurred between creation and deployment.
Maintain Audit Trails and Logging
Log every push, pull, deletion, and tag event.
Monitor logs for suspicious activity (e.g., repeated failed login attempts, downloads outside business hours).
Retain logs for a specified period to support troubleshooting and compliance audits.
Keep the Registry Software Updated
Regularly apply patches and updates to the registry software itself.
Outdated registry services can introduce vulnerabilities even if your container images are secure.
Integrate with Your CI/CD Pipeline
Automate the process of scanning and promoting images from development to production.
Configure your pipeline so that only “clean” images (i.e., scanned and approved) move on to the next environment.
Enforce policies (e.g., block images with critical vulnerabilities) to maintain a high security baseline.
Enforce Tagging and Versioning Conventions
Adopt consistent naming and tagging practices for images (e.g., my-app:2.1.0 or my-app:latest).
Clear conventions prevent confusion about which images are safe or tested and reduce the risk of deploying the wrong version.
By implementing these measures, organizations can better protect against malicious attacks, ensure compliance with industry regulations, and maintain a more reliable containerized infrastructure. Tools such as Mirantis Secure Registry or other enterprise-grade registries incorporate many of these features out of the box, simplifying the path toward a secure software supply chain.
What to look for in a container registry
A private registry performs several crucial functions. It empowers an organization to include only approved images in the registry. It helps the organization control access, and may also provide the ability to scan the registry contents for known vulnerabilities. Additionally, enterprises seeking a private container registry should look for the following features:
Binary-level image scanning from a regularly updated vulnerability database. Critical vulnerabilities are often hidden well below the surface in container images—in the binaries and libraries used by dependencies for your core processes, for example. It’s important that your registry enables “deep-tissue” scanning down to the binary level—and that your scan is backed up by regular database updates from an expert team.
Secure software supply chain through tight integration with CI/CD tools and processes. The more closely your registry works with your Continuous Integration and Continuous Delivery solutions, the more you can automate processes like image promotion for simpler, faster, and more secure software lifecycles.
Deployment flexibility. How flexibly can you use your container registry on-prem, on private clouds, on public clouds, or in multi-cloud environments? Will it be able to adapt as your architecture evolves?
Our own Mirantis Secure Registry provides these features with installation to any Kubernetes distribution via Helm, a vulnerability database updated daily, close CI/CD integration, and more. To explore Mirantis Secure Registry with a trial license, click here.
