How to Configure Credentials for GCR Proxy Repository

As documented in https://cloud.google.com/container-registry/docs/advanced-authentication, GCR offers a number of authentication methods, but to configure authentication for a GCR proxy repository in Nexus 3, the JSON Key File option should be used. 

To do this, via the Nexus 3 UI --> Repositories --> 'GCR PROXY REPO ID' --> HTTP --> Authentication, set:

• Authentication Type: 

  Username

• Username: 

  _json_key

• Password: 

  contents of keyfile.json