Create a build source that can be linked to one or more
services in the same workspace.

Exactly one of git or image must be set:

• git: the build source is git-backed. The code is built in
  the requested region (defaults to oregon).
• image: the build source is image-backed. It points at an
  existing image in an external registry; no build is performed.

envVars, secretFiles, and envGroupIds set the build-time
environment and are only valid for git sources; the request fails
if any are provided with image.