Hmm... playing with the docker functionality again (massive progress for us!) one of the arguments we were passing was

--build-arg CONTAINER_GID={os.getgid()}

. (this was for a development container so that it ran as the existing user rather than as root or some fixed user ID). It's not clear how to do this with

extra_build_args

in pants (or generally, how to pass in a variable like that). I obviously can't

import os

to call `getgid`; I could pass it in as an env variable, but I can't see how to use env variables within a

BUILD

right now. Possible or not?