I think that (at least) 3 common definitions should be provided for https://github.com/stac-extensions/processing#expression-object to better guide users.

• a generic "python script entry points" similar to what setup(entry_point=...) or poetry.scripts expect
  (e.g.: my_package.my_module:some_function or my_package.my_module:SomeClass.a_method)

• a URI based reference to a script or executable
  (somewhat relates to Add a Software object #29 about the href requirement)
  (for this, I would suggest that "expression": {"href": "<uri>", "type"; "<media-type>"} is used, since expression allows anything)

• some docker container or similar, using an expression like ghcr.io/NAMESPACE/IMAGE_NAME:latest (or a more explicit docker run call)