Try .cmd file extension suffix for credential helpers on Windows

On Windows, gcloud auth configure-docker generates the following section in ~/.docker/config.json to specify that the cred helper is docker-credential-gcloud.

"credHelpers": {
    "gcr.io": "gcloud",
    "us.gcr.io": "gcloud",
    "eu.gcr.io": "gcloud",
    "asia.gcr.io": "gcloud",
    "staging-k8s.gcr.io": "gcloud"
},

The actual binary to run is docker-credential-gcloud.cmd. Without .cmd, Jib doesn’t seem able to run the binary.

[WARNING] The system does not have docker-credential-gcloud CLI
[WARNING]   Caused by: Cannot run program "docker-credential-gcloud": CreateProcess error=2, The system cannot find the file specified

docker-compose suffered from the same issue, and the workaround was to manually change gcloud to gcloud.cmd in ~/docker.config.json. OTOH, apparently, Docker CLI has been working without .cmd.

[matthewmichihara]

Confirmed that both of those flags work as expected:

[INFO] Using credential helper docker-credential-gcloud for gcr.io/spokane/untitled

[INFO] Using credential helper docker-credential-gcloud.cmd for gcr.io/spokane/untitled

[matthewmichihara]

great, thanks!