podman compose vs docker compose

My podman-fu is pretty rusty :-( I'm following https://docs.mattermost.com/install/install-docker.html but using podman. I've fixed enough differences to get the containers up and running, but the mattermost container immediately fails with:

Error: failed to load configuration: could not create config file: open /mattermost/config/config.json: permission denied

How can I dig into this? If I run the container with -it and open a shell, I can write into that directory. Something seems to be preventing it from writing out it's config.