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/java/.dockerignore

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

examples/java/src/main/java/generated_program/App.java

@@ -185,9 +185,6 @@ FROM alpine
 RUN echo "This uses an inline Dockerfile! 👍"
                 """)
                 .build())
-            .context(BuildContextArgs.builder()
-                .location("./app")
-                .build())
             .build());
 
         var dockerLoad = new Image("dockerLoad", ImageArgs.builder()