Housekeeping (#8)

* Rename the provider to dockerbuild.
* Add Makefile targets required by CI.
* Add per-language test targets compatible with CI.
* Fix broken example yaml.
* Add gitignore exclusions to fix SDK generation in CI.
* Fix lint errors.
* Vendor pulumi CLI via `tools.go` instead of `.pulumi`.
* Consolidate `go.mod`s into one file, with a minimal `go.mod` for the
Go SDK.
* Add codecov.

sdk/go/dockerbuild/pulumi-plugin.json

@@ -0,0 +1,5 @@
+{
+  "resource": true,
+  "name": "dockerbuild",
+  "server": "github.com/pulumi/pulumi-dockerbuild"
+}