I could have sworn the context property was required, but evidently it
isn't and we weren't handling the case when it was missing.
This PR updates things to set a default location of the current
directory if the context is absent. Some unit tests are also added.
Fixes#78.
* 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.
