Run mypy over docker directory

[DMRobertson]

There's a couple of scripts in there which it would be nice to typecheck.

• change mypy.ini's files list to include docker
• run mypy and see what errors crop up
• fix them until none remain

There's the contrib directory too, but we don't promise to support that diligently.

[DMRobertson]

#12280 was a great start! I'd like to see this pass with disallow_untyped_defs = True too. But I couldn't see how to setup the config in a way which would apply to the docker directory. Perhaps something like #11302 would help?