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.

sdk/python/pulumi_docker_build/image.py

@@ -852,7 +852,7 @@ class Image(pulumi.CustomResource):
             context={
                 "location": "app",
                 "named": {
-                    "golang_latest": {
+                    "golang:latest": {
                         "location": "docker-image://golang@sha256:b8e62cf593cdaff36efd90aa3a37de268e6781a2e68c6610940c48f7cdf36984",
                     },
                 },
@@ -1290,7 +1290,7 @@ class Image(pulumi.CustomResource):
             context={
                 "location": "app",
                 "named": {
-                    "golang_latest": {
+                    "golang:latest": {
                         "location": "docker-image://golang@sha256:b8e62cf593cdaff36efd90aa3a37de268e6781a2e68c6610940c48f7cdf36984",
                     },
                 },