This also necessitated a few refactors:
- Replace __wrap_docker_exec() with straightforward implementations
- Move error message into DockerExecuteError.__init__
- Refactor out common behavior into _get_image_config()

Also, change git_describe() to return a GitDescribe dataclass result.

Other notable changes:
- ScubaAlias, ScubaVolume are now frozen dataclass classes
- Added Loader._rooted_loader to satisfy requirement that yaml.load's
  second parameter must be a Type
- Added _get_str()
- Added 'default' parameter to _get_typed_val()

Notable changes:
- Made ScubaContext a dataclass with non-optional members. This required
  significant changes in ScubaContext.process_command(), since all
  values must be known when the object is constructed, and not default
  to None to be updated later in the function.
- ScubaContext.process_command() image and shell arguments were renamed
  to add _override to clarify their behavior.

Notable gaps:
- open_scubadir_file() returns Any because I couldn't figure out how to
  correctly annotate a TextIO object with an additional container_path
  attribute.

Move argcomplete stub to its own module.