feat: image build args

[cylewitruk]

What did you implement:

• Adds support for buildargs argument when building an image. This is exposed on the ImageBuildOpts builder as build_args().
• The default test Dockerfile content has been appended with an ARG which is used in tests.
• Minor boyscouting (fixed a few warnings).

Closes: #72

How did you verify your change:

New test: image_create_with_build_args().
Also tested within my local app where I need this and it worked just fine.

What (if anything) would need to be called out in the CHANGELOG for the next release:

It's just a minor feature, but maybe good to add that it now works? (was a TODO previously).