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/yaml/Pulumi.yaml

@@ -166,8 +166,6 @@ resources:
         inline: |
           FROM alpine
           RUN echo "This uses an inline Dockerfile! 👍"
-      context:
-        location: "./app"
 
   # docker buildx build --load .
   dockerLoad: