In this lesson, you’ll learn how to standardize your organization’s development environment by creating custom builder images for Cloud Native Buildpacks (CNB).
While standard Buildpacks are great for getting an app running quickly, large organizations often need to enforce specific security policies—like using a corporate-approved version of a language runtime or including pre-configured monitoring agents. Instead of letting every team pick their own tools, you can create a Custom Builder Image that acts as the "gold standard" for all your builds.
What is a Builder Image?
A Builder Image is a container image that bundles together everything needed to detect, build, and package your application code into a runnable container. It typically contains:
- Buildpacks: The logic that transforms source code into an image.
- Lifecycle: The binary that manages the build phases (detect, analyze, build, export).
- Base Stack: The OS layer (like Ubuntu or a minimal distroless image) that the final app will run on.
Enforcing Standards
To customize this, you create a builder.toml configuration file. This file tells the builder which buildpacks to include and which base images to use for the app’s runtime environment.
For example, you might create a custom builder that forces the use of a specific, hardened base image:
[stack]
build-image = "my-registry.com/hardened-build-base:latest"
run-image = "my-registry.com/hardened-run-base:latest"
[[buildpacks]]
uri = "docker://my-registry.com/company-java-buildpack:v1.2"
By pushing this builder.toml and your selected buildpacks into a single image, you provide your developers with a standardized "black box." When a developer runs the pack command using your custom builder, the build automatically complies with your security requirements—such as ensuring no root users are allowed or that a specific vulnerability-scanned base image is always used.
This approach ensures that every application across the enterprise is built consistently, reducing configuration drift and making it significantly easier to audit the entire software supply chain.
Key Takeaway: Custom builder images act as a centralized blueprint that enforces organizational security and operational standards automatically during the container build process.