Initial provider implementation (#18)

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

examples/java/Pulumi.yaml

@@ -1,6 +1,10 @@
-name: provider-dockerbuild
+name: provider-docker-build
 runtime: java
+config:
+  dockerHubPassword:
+    type: string
+    secret: true
 plugins:
   providers:
-    - name: dockerbuild
+    - name: docker-build
       path: ../../bin