Just spent an hour and a half puzzling over why this variable in my compose.yml wasn't working only to finally discover someone pointing out that it doesn't actually get set properly in its corresponding config file and you have to do it manually. Which meant finding the default volume and inadvisably editing it outside of the container because whoever put the Docker image together didn't see fit to include it with any text editor at all. And this was not a small project, this was fucking Collabora.
I respect #
FOSS devs; I'm not much of a programmer myself, but sometimes, they baffle me for entirely different reasons.