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Builders

This document defines Builder images and the conventions to which they are expected to adhere.

What is a Builder?

A Builder image is a special classification for images that run as a part of the Build CRD's steps:.

For example, in the following Build the images, gcr.io/cloud-builders/gcloud and gcr.io/cloud-builders/docker are "Builders".:

spec:
  steps:
  - image: gcr.io/cloud-builders/gcloud
    ...
  - image: gcr.io/cloud-builders/docker
    ...

Typical Builders

A Builder is typically a purpose-built container whose entrypoint is a tool that performs some action and exits with a zero status on success. These entrypoints are often command-line tools, for example, git, docker, mvn, and so on.

Typical builders set their command: (aka ENTRYPOINT) to be the command they wrap and expect to take args: to direct their behavior.

See here and here for more builders.

Atypical Builders

It it possible, although less typical to implement the Builder convention by overriding command: and args: for example:

steps:
- image: ubuntu
  command: ['/bin/bash']
  args: ['-c', 'echo hello $FOO']
  env:
  - name: 'FOO'
    value: 'world'

Specialized Builders

It is also possible for advanced users to create purpose-built builders. One example of this are the "FTL" builders.

What are the Builder conventions?

Builders should expect a Build to implement the following conventions:

/workspace: The default working directory will be /workspace, which is a volume that is filled by the source: step and shared across build steps:.
/builder/home: This volume is exposed to steps via $HOME.
Credentials attached to the Build's service account may be exposed as Git or Docker credentials as outlined here.

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