The alias directive does not override the value for $document_root, which you have specified in your server block to be /usr/share/nginx/html/myserver. You need to replace alias with root to change the value of $document_root within the location block for FreshRSS.
user460778 answer is the correct one, there can be only entry point for a container, here it should be php-fpm service, So you need to run crons from supervisor. My supervisor file looks like this
command = /usr/local/sbin/php-fpm
Do not set your system hostname to your naked domain name. Do not set Postfix's mailname to the naked domain name. Do not add the naked domain name to Postfix's mydestination, or to any other Postfix configuration option. Doing any of these will cause Postfix to try to deliver the mail locally rather than sending it out to the Internet.
Name the system with ...