Initial provider implementation (#18)

This brings over the initial buildx prototype from pulumi/pulumi-docker
and fixes various build and release issues.

sdk/dotnet/pulumi-plugin.json

@@ -1,5 +1,4 @@
 {
   "resource": true,
-  "name": "dockerbuild",
-  "server": "github.com/pulumi/pulumi-dockerbuild"
+  "name": "docker-build"
 }