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/nodejs/.dockerignore

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

examples/nodejs/index.ts

@@ -154,9 +154,6 @@ const inline = new docker_build.Image("inline", {
 RUN echo "This uses an inline Dockerfile! 👍"
 `,
     },
-    context: {
-        location: "./app",
-    },
 });
 const dockerLoad = new docker_build.Image("dockerLoad", {
     push: false,