Collabora Online and Nextcloud, another try.

I swear it should be the other way round (it looks to me like Nextcloud
is embedding the code iframe), but disabling CSP on Nextcloud worked and
I saw a comment on the Internet about it so lets give it a try.

conf.d/nextcloud.shore.co.il.conf

@@ -21,6 +21,9 @@ server {
     include     snippets/ssl.conf;
     include     snippets/nextcloud-well-known.conf;
 
+    # Allow to include iframes in code.shore.co.il.
+    add_header  Content-Security-Policy "frame-ancestors https://code.shore.co.il";
+
     location / {
         proxy_pass              http://$nextcloud$request_uri;
         proxy_http_version      1.1;