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/cli.go

@@ -141,6 +141,10 @@ func (c *cli) Err() *streams.Out {
 	return streams.NewOut(&c.err)
 }
 
+func (c *cli) SupportsMultipleExports() bool {
+	return c.host.supportsMultipleExports
+}
+
 // rc returns a registry client with matching auth.
 func (c *cli) rc() *regclient.RegClient {
 	hosts := []config.Host{}