By allowing you to define a specific builder image responsible for the entire build process, OKD’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.

Custom Builder Image

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

Table 1. Custom Builder Environment Variables
Variable Name Description


The entire serialized JSON of the Build object definition. If you need to use a specific API version for serialization, you can set the buildAPIVersion parameter in the custom strategy specification of the build configuration.


The URL of a Git repository with source to be built.


Uses the same value as SOURCE_REPOSITORY. Either can be used.


Specifies the subdirectory of the Git repository to be used when building. Only present if defined.


The Git reference to be built.


The version of the OKD master that created this build object.


The Docker registry to push the image to.


The Docker tag name for the image being built.


The path to the Docker credentials for running a docker push operation.


Specifies the path to the Docker socket, if exposing the Docker socket was enabled in the build configuration (if exposeDockerSocket was set to true.)

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 OKD 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.

Custom Builder Workflow

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 OKD:

  1. The Build object definition contains all the necessary information about input parameters for the build.

  2. Run the build process.

  3. If your build produces an image, push it to the build’s output location if it is defined. Other output locations can be passed with environment variables.