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/python/__main__.py

@@ -141,9 +141,6 @@ inline = docker_build.Image("inline",
         inline="""FROM alpine
 RUN echo "This uses an inline Dockerfile! 👍"
 """,
-    ),
-    context=docker_build.BuildContextArgs(
-        location="./app",
     ))
 docker_load = docker_build.Image("dockerLoad",
     push=False,