Hi,
I have selfhosted piwigo on raspi4 using docker. I too faced this issue (after I installed videojs plugin - not sure this is relevant). But I somewhat resolved it by increasing timeout values in following files. I say somewhat because it still gives me 504 errors but not always as before.

Note : I am not an expert in PHP or webadmin but found some info through net. Kindly check if it works for you. If not, maybe someone else can help.

File :
nginx/site-confs/default
add fastcgi_read_timeout 300; in location section. E.g. :
eg.
    location ~ \.php$ {
        fastcgi_split_path_info ^(.+\.php)(/.+)$;
        # With php5-cgi alone:
        fastcgi_pass 127.0.0.1:9000;
        # With php5-fpm:
        #fastcgi_pass unix:/var/run/php5-fpm.sock;
        fastcgi_index index.php;
        fastcgi_read_timeout 300;
        include /etc/nginx/fastcgi_params;

    }

File : /etc/php7/php-fpm.d/www.conf
added line : request_terminate_timeout = 300

Also in :
File : /etc/nginx/nginx.conf
Add following in http{..} section

http {
    #...
        fastcgi_read_timeout 300;
    #...

restarted the containers and thats it.