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/provider.go

@@ -55,8 +55,8 @@ func (c *Config) Annotate(a infer.Annotator) {
 }
 
 // Configure validates and processes user-provided configuration values.
-func (c *Config) Configure(_ context.Context) error {
-	h, err := newHost(c)
+func (c *Config) Configure(ctx context.Context) error {
+	h, err := newHost(ctx, c)
 	if err != nil {
 		return fmt.Errorf("getting host: %w", err)
 	}