Now based on Alpine Linux and s6 as supervisor, resulting in a much smaller Docker image. See README.md for details and other additions.

root/etc/services.d/nginx/run

@@ -1,2 +1,2 @@
-#!/bin/sh
+#!/usr/bin/with-contenv sh
 exec nginx

root/etc/services.d/php/run

@@ -1,2 +1,2 @@
-#!/bin/sh
+#!/usr/bin/with-contenv sh
 exec php-fpm

root/etc/services.d/ttrss-daemon/run

@@ -1,7 +1,7 @@
-#!/bin/sh
+#!/usr/bin/with-contenv sh
 
 while true; do
   cd /var/www/ttrss
-  php -f /var/www/ttrss/update_daemon2.php
+  exec s6-setuidgid www-data php -f /var/www/ttrss/update_daemon2.php
   sleep 5m
 done

root/etc/services.d/ttrss-updater/run

@@ -1,6 +1,3 @@
-#!/bin/sh
-
-while true; do
-  /srv/update-ttrss.sh
-  sleep 24h
-done
+#!/usr/bin/with-contenv sh
+sleep 24h
+exec s6-setuidgid www-data /srv/update-ttrss.sh --wait-exit 24h