Do not diff __internal property in DiffConfig (#414)

This PR updates the implementation of DiffConfig to remove the
`__internal` property when comparing provider config between old state
and new input. It is observed that the `__internal` property is present
when a default provider is used. This causes a provider replacement
every time a user upgrades pulumi-docker-build, as the `__internal`
property is always stripped from state, so Pulumi will always report a
diff.

Ideally, we should determine if this bug can be fixed in either upstream
pulumi-go-provider or pulumi itself. For now, we will fix this locally
within pulumi-docker-build to unblock our users.

Changes:

- Update DiffConfig implementation to strip `__internal` property from
new inputs.
- Add a nodejs upgrade test to test that this solution works

Fixes: #404

examples/upgrade-node/index.ts

@@ -0,0 +1,11 @@
+import * as pulumi from "@pulumi/pulumi";
+import * as docker_build from "@pulumi/docker-build";
+
+const inline = new docker_build.Image("inline", {
+  push: false,
+  dockerfile: {
+    inline: `FROM alpine
+RUN echo "This uses an inline Dockerfile! 👍"
+`,
+  },
+});