Change dmypy status file with config or environment variable

Feature

The only way I've been able to set the dmypy status file is by passing --status-file on the command-line, but I'm running dmypy in a few different scripts and would love to be able to hard-code this in one spot.

Pitch

Instead of dmypy --status-file /tmp/dmypy-status, I'd like to either make an environment variable (DMYPY_STATUS_FILE, DMYPY_STATUS_FILE, or similar) or even a mypy config option (status_file?) so that I don't have to explicitly set the status file in every command-line invocation.

Why are you even changing the status file?

Great question. I'm using Docker with a host-mounted volume, and I'd like to avoid sharing the host-mounted volume back and forth between the host and the container. By moving it out of the current directory, I'll be able to run dmypy both inside and outside of the container without conflicts.

Start at dmypy’s handling of the --status-file command-line option and trace how configuration and environment values are loaded. Decide which supported mechanism fits the request, then verify that users can set the status file once and invoke dmypy without repeating the option, including separate host and container runs.