To try to clarify, I'm proposing this modeling:

class DockerSourceField(SingleSourceField):
   default = "Dockerfile"

class SyntheticDockerSourceField(DockerSourceField):
  default = None
   expected_num_files = 0
   alias = "_source"

Then your rules use

tgt.get(DockerSourceField)

and are agnostic to knowing

SyntheticDockerSourceField

exists, beyond needing to set

enable_codegen=True

when you're hydrating sources