Chapter 6. Custom Builder

By allowing you to define a specific builder image responsible for the entire build process, OpenShift Container Platform’s Custom build strategy was designed to fill a gap created with the increased popularity of creating container images. When there is a requirement for a build to still produce individual artifacts (packages, JARs, WARs, installable ZIPs, and base images, for example), a Custom builder image using the Custom build strategy is the perfect match to fill that gap.

A Custom builder image is a plain container image embedded with build process logic, for example for building RPMs or base container images. The openshift/origin-custom-docker-builder image is available on the Docker Hub as an example implementation of a Custom builder image.

Additionally, the Custom builder allows implementing any extended build process, for example a CI/CD flow that runs unit or integration tests.

To fully leverage the benefits of the Custom build strategy, you must understand how to create a Custom builder image that will be capable of building desired objects.

Upon invocation, a custom builder image will receive the following environment variables with the information needed to proceed with the build:

It is recommended that you write your Custom builder image to only read the information provided by the BUILD environment variable, which conforms to an OpenShift Container Platform API version. You can then parse the required information from the build definition JSON instead of relying on the other individual environment variables provided to the builder image. However, the individual environment variables are provided for convenience if you do not want to parse the build definition.

Although Custom builder image authors have great flexibility in defining the build process, your builder image must still adhere to the following required steps necessary for seamlessly running a build inside of OpenShift Container Platform: