how to use foundry vtt data volume with user namespaces

[hemmick]

I've followed the steps at this helpful link to create a user 'dockremap' who exists in my /etc/docker/daemon.json as-below:

{
        "userns-remap": "dockremap",
        "experimental": false,
        "live-restore": true,
        "ipv6": false,
        "no-new-privileges": true
}

additionally I have /etc/subuid dockremap:<dockremapsuid>:65536 and subgid dockremap:<dockremapsgid>:65536

Finally -- I have built my docker container with your prebuilt image command and am trying to run it with the following command:
docker run --publish 30000:30000/tcp --volume <my_homedir>/vttdata:/data fvtt-prebuilt:0.7.9

The intent is to run foundry vtt in a container, but to persist my data in my home directory. However, when I run the above, I get the following error:

Entrypoint | 2021-01-06 00:52:24 | [info] Setting data directory permissions.
chown: /data: Operation not permitted
chown: /data/Config: Operation not permitted

It seems that the docker container might be trying to chown data -- I have already confirmed that <my_homedir>/vttdata is chowned to 'dockremap:dockremap'

Any idea what I might be missing, here?

[felddy]

I haven't played with remapping yet so I can't reproduce your setup here...

I'd take a look at setting FOUNDRY_UID and FOUNDRY_GID environment variables to match the values you are using in remap:
https://github.com/felddy/foundryvtt-docker#optional

Optionally, you can disable the chown step entirely by setting the CONTAINER_PRESERVE_OWNER variable to /data.