Docker 1.14 credential helper support

Docker client v1.14 introduces registry-specific credential helpers. docker-credential-gcr will now preferentially act as a credential helper on Docker client versions 1.14+ and as a general credential store on 1.11 - 1.13.

Bugfix related to 3p credential storage

1.2.0 -> 1.2.1

Change-Id: I73df4864d35812b2615ef4ecc556103b628ec5cc
Signed-off-by: Jake Sanders <jsand@google.com>

Minor config change and bugfix

You must now use "gcloud" rather than "gcloud_sdk" to reference gcloud as a token source.

Take control!

Added 2 new user controls for docker-credential-gcr:

• docker-credential-gcr config: Configure where (and in which order) the credential helper searches for GCR credentials.
• docker-credential-gcr clear: Remove ALL stored credentials. Handier than specifically gcr-logouting followed by several erases

Bug fixes

This release contains support for beta.gcr.io, as well as a fix where 'docker build' was not pulling GCR's credentials.

Release!

Please pick the archive which matches your OS and CPU architecture.