Building an image on the top of nasqueron/nginx-php-fpm Docker image, I see:
$ docker run --name tmp 585a158865e9 /usr/local/sbin/runsvdir-init: line 2: /usr/local/etc/envvars: Permission denied
At this stage, this image ran as app, not as root, and /usr/local/etc/envvars didn't exist.
What's going on is the export > /usr/local/etc/envvars fails, at the user app doesn't have write access to /usr/local/etc.
This part of the image should be run as root, so we could detect the id and offer:
The first seems preferable, as nginx needs to listen port 80.
This only occur when a USER app ; <commands ; USER root sequence is interrupted when we inspect the container at this step.