Just to document what I did for future visitors, I have manually added the options suggested in the topics linked in my previous message.
client_header_buffer_size 64k;
large_client_header_buffers 4 64k;
The values might be different for other users, but they look more than enough for my needs.
These options shall be manually added to the /etc/nginx/conf.d/discourse.conf NGINX configuration file, and after having restarted nginx service in the container, the problem will be solved.
Obviously this has to be redone manually at every container rebuild, unless this setting is added to the official web templates.