On the server that runs pixel, the pixel job is run by the user pixel. However, when you run Pixels docker containers, the output (files) are owned by root and the pixel script (started by the user pixel) do not have write access to the files.

For now, the pixel user runs the script as su.

I've used a hack like this https://github.com/sitespeedio/sitespeed.io/blob/main/docker/scripts/start.sh#L43-L46 before but maybe there's a better/newer way to fix it.