Fix a panic that could occur when context was omitted (#83)

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.

examples/go/.dockerignore

@@ -0,0 +1,2 @@
+command-output
+tmp

examples/go/main.go

@@ -207,9 +207,6 @@ func main() {
 			Dockerfile: &dockerbuild.DockerfileArgs{
 				Inline: pulumi.String("FROM alpine\nRUN echo \"This uses an inline Dockerfile! 👍\"\n"),
 			},
-			Context: &dockerbuild.BuildContextArgs{
-				Location: pulumi.String("./app"),
-			},
 		})
 		if err != nil {
 			return err