Pull registry credentials from Docker config.

[coollog]

This should be used if credentials could not be found in defined credHelpers and Maven settings, but before inferring credential helpers.

---

UPDATED by @chanseokoh to reflect the current state of Jib handling ~/.docker/config.json.

Docker config may store credentials in 3 ways (or a combination):

1. Credential helpers for targeted registries

{
    "credHelpers": {
        "registry.example.com": "credential helper binary (suffix to docker-credential-)",
        "gcr.io": "gcloud",
        ...
    },
    ...
}

2. Legacy "default" credential helper. For recent Docker versions, consider using credHelpers only.

{
    "credsStore": "credential helper binary (suffix to docker-credential-)",
    ...
}

3. Explicitly embedding password in plain text (discouraged). Consider using a credential helper binary.

{
    "auths": {
        "registry": {
            "auth": "username:password string in base64",
        },
        ...
    }
}

[coollog]

Part of #22