Add support for multiple exporters (#235)

Buildkit 0.13 introduced support for [multiple
exporters](https://docs.docker.com/build/exporters/#multiple-exporters).
We currently return an error in these situations.

Instead, inspect the builder's version when loading the node and relax
this error if we see it's running at least 0.13.

Fixes https://github.com/pulumi/pulumi-docker-build/issues/21.

provider/internal/client.go

@@ -58,6 +58,8 @@ type Client interface {
 	ManifestCreate(ctx context.Context, push bool, target string, refs ...string) error
 	ManifestInspect(ctx context.Context, target string) (string, error)
 	ManifestDelete(ctx context.Context, target string) error
+
+	SupportsMultipleExports() bool
 }
 
 // Build encapsulates all of the user-provider build parameters and options.
@@ -88,8 +90,6 @@ func newDockerCLI(config *Config) (*command.DockerCli, error) {
 		return nil, err
 	}
 
-	// TODO: Log some version information for debugging.
-
 	return cli, nil
 }