Nginx cannot read response from fastcgi when user id changed by environment variable

[Write]

[crit] 61#61: *598 open() "/var/lib/nginx/tmp/fastcgi/7/00/0000000007" failed (13: Permission denied) while reading upstream

User owning the folder is still "100" instead of the one set in PUID/GUID var.
This lead to file >= 10MB not being served and throwing this error.