`[GLOBAL].docker_execution` can't be passed on the CLI

[thejcannon]

Describe the bug

$ pants --docker-execution test ::
Unknown flag --execution on docker scope

Pants version
2.20

OS
Linux

Additional info
N/A.

[huonw]

I imagine this is a conflict with the pants.backend.docker backend? Can you confirm with a minimal reproducer?

[thejcannon]

Yes that is the case.

[kaos]

The confirmation is in the error message:

Unknown flag --execution on docker scope

The docker scope is from the docker subsystem.