Changes

fix(tekton/v1/tasks): fix image build task for more common stories (#3916)

- Add `CREDS_DIR` env and script logic to locate docker credentials and
GSA key, populate /kaniko/.docker/config.json and set
`GOOGLE_APPLICATION_CREDENTIALS`
- Move common container image into stepTemplate and remove per-step
image fields.
 (commit: 23a8046)