test_build_default_launches was using cibuildwheel setup.cfg which led to the test failing once python 3.6 was dropped.
Use an empty package, provided by fake_package_dir, to run the test.
OCIContainer.container_engine -> OCIContainer.engine
OCIContainer.docker_image -> OCIContainer.image
Variables that refer to 'docker_images' have been renamed to
'container_images'
* Add test for defaults, fix platform detection
* Improve correctness of reader.identifier with block
* Fix options test to be multiplatform
* 'Refactored by Sourcery'
docs: write a section on overrides
tests: test docker launches
feat: add identifiers to launches
test: add test for correct build step generation
Signed-off-by: Henry Schreiner <henryschreineriii@gmail.com>