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/dotnet/Program.cs

@@ -239,10 +239,6 @@ COPY hello.c ./
 RUN echo ""This uses an inline Dockerfile! 👍""
 ",
         },
-        Context = new DockerBuild.Inputs.BuildContextArgs
-        {
-            Location = "./app",
-        },
     });
 
     var dockerLoad = new DockerBuild.Image("dockerLoad", new()