No-op during Read if credentials are invalid (#194)

The `Image` resource already ignores errors during read but the `Index`
resource was requiring the inspect call to always succeed. This will often 
fail due to https://github.com/pulumi/pulumi/issues/4981 (note however in
this case credentials are stored with the resource instead of the provider).

This changes our logic to instead emit a warning if the credentials are
invalid.

Fixes https://github.com/pulumi/pulumi-docker-build/issues/121.

provider/internal/image.go

@@ -802,6 +802,7 @@ func (i *Image) Read(
 			continue
 		}
 
+		//nolint:gocritic // Bytes aren't copied in a hot path.
 		for _, d := range descriptors {
 			if d.Platform != nil && d.Platform.Architecture == "unknown" {
 				// Ignore cache manifests.