vendor in docker-credential-gcr

[jamesward]

This vendors in docker-credential-gcr so that no special service accounts are needed to publish to GCR. This is similar to how the CNCF Buildpacks vendor in the creds helpers: https://github.com/buildpack/lifecycle/blob/master/images/build/Dockerfile#L6-L8

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: jamesward To fully approve this pull request, please assign additional approvers. We suggest the following additional approver: mattmoor

If they are not already assigned, you can assign the PR to them by writing /assign @mattmoor in a comment when ready.

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS

Approvers can indicate their approval by writing /approve in a comment Approvers can cancel approval by writing /approve cancel in a comment

[imjasonh]

/ok-to-test

[imjasonh]

Should we also do this for the jib-gradle template?

cc @loosebazooka