Trouble with Nextcloud FPM image in Docker

I’m quite new to Docker, and have run up against an issue that I’m hoping someone here has a straightforward fix for.
I’m hosting my containers on a Synology NAS, and as such some images (eg. Nextcloud) that have an image option with a built-in Apache web server won’t work due to a kernel issue. They fail with a reference to the random number generator.
This obliges me to use the FPM image for Nextcloud, which requires another container to act as the web server for the content.
My issue: The application and database containers look all right but when the web container attempts to access the application container I get this error:

2022/07/12 23:04:41 [error] 34#34: *2 “/var/www/html/index.php” is forbidden (13: Permission denied), client: 172.21.0.1, server: , request: “GET / HTTP/1.1”, host: “10.33.10.2:9095”

I seem to need to find a way of setting the permissions on the content and / or defining the user with which the web container is trying to access the content.

The NAS is running DSM 7.0.1-42218 and Docker version 20.10.3-1239. I’m managing the environment with Portainer. Here is the docker compose config from the stack:

version: '2.4'

volumes:
  ncapp:

networks:
  inside:
  outside:
    external: true
    name: PROXYBRIDGE #Access to site from a reverse proxy

services:
  ncdb: #Database Container
    image: mariadb:latest
    restart: always
    command: --transaction-isolation=READ-COMMITTED --binlog-format=ROW
    volumes:
      - /volume1/docker/ncdb:/var/lib/mysql #Bind Mount for database files
    environment:
      - MYSQL_ROOT_PASSWORD=nextcloud
      - MYSQL_PASSWORD=nextcloud
      - MYSQL_DATABASE=nextcloud
      - MYSQL_USER=nextcloud
      - TZ=Pacific/Auckland
    networks:
      - inside

  ncapp: #Application Container
    image: nextcloud:10-fpm
    restart: always
    depends_on:
      - ncdb
    links:
      - ncdb
    volumes:
      - ncapp:/var/www/html #Named Volume for application data
      - /volume1/docker/ncapp/data:/var/www/html/data #Bind Mount for cloud storage of User Data
    environment:
      - MYSQL_PASSWORD=nextcloud
      - MYSQL_DATABASE=nextcloud
      - MYSQL_USER=nextcloud
      - MYSQL_HOST=ncdb
      - NEXTCLOUD_ADMIN_PASSWORD=XXXX
      - NEXTCLOUD_ADMIN_USER=XXXX
    networks:
      - inside

  ncweb: #Web front end Container
    image: nginx:latest
    restart: always
    depends_on:
      - ncapp
    ports:
      - 9095:80
    links:
      - ncapp
    volumes:
      - /volume1/docker/ncweb/nginx.conf:/etc/nginx/nginx.conf:ro #Web server config file
    volumes_from:
      - ncapp:ro #Access to Application Data
    networks:
      - inside
      - outside

And here is the config file for the Nginx web front end:

worker_processes auto;

error_log  /var/log/nginx/error.log warn;
pid        /var/run/nginx.pid;


events {
    worker_connections  1024;
}


http {
    include       /etc/nginx/mime.types;
    default_type  application/octet-stream;

    log_format  main  '$remote_addr - $remote_user [$time_local] "$request" '
                      '$status $body_bytes_sent "$http_referer" '
                      '"$http_user_agent" "$http_x_forwarded_for"';

    access_log  /var/log/nginx/access.log  main;

    sendfile        on;
    #tcp_nopush     on;

    # Prevent nginx HTTP Server Detection
    server_tokens   off;

    keepalive_timeout  65;

    #gzip  on;

    upstream php-handler {
        server ncapp:9000;
    }

    server {
        listen 80;

        # HSTS settings
        # WARNING: Only add the preload option once you read about
        # the consequences in https://hstspreload.org/. This option
        # will add the domain to a hardcoded list that is shipped
        # in all major browsers and getting removed from this list
        # could take several months.
        #add_header Strict-Transport-Security "max-age=15768000; includeSubDomains; preload;" always;

        # set max upload size
        client_max_body_size 512M;
        fastcgi_buffers 64 4K;

        # Enable gzip but do not remove ETag headers
        gzip on;
        gzip_vary on;
        gzip_comp_level 4;
        gzip_min_length 256;
        gzip_proxied expired no-cache no-store private no_last_modified no_etag auth;
        gzip_types application/atom+xml application/javascript application/json application/ld+json application/manifest+json application/rss+xml application/vnd.geo+json application/vnd.ms-fontobject application/x-font-ttf application/x-web-app-manifest+json application/xhtml+xml application/xml font/opentype image/bmp image/svg+xml image/x-icon text/cache-manifest text/css text/plain text/vcard text/vnd.rim.location.xloc text/vtt text/x-component text/x-cross-domain-policy;

        # Pagespeed is not supported by Nextcloud, so if your server is built
        # with the `ngx_pagespeed` module, uncomment this line to disable it.
        #pagespeed off;

        # HTTP response headers borrowed from Nextcloud `.htaccess`
        add_header Referrer-Policy                      "no-referrer"   always;
        add_header X-Content-Type-Options               "nosniff"       always;
        add_header X-Download-Options                   "noopen"        always;
        add_header X-Frame-Options                      "SAMEORIGIN"    always;
        add_header X-Permitted-Cross-Domain-Policies    "none"          always;
        add_header X-Robots-Tag                         "none"          always;
        add_header X-XSS-Protection                     "1; mode=block" always;

        # Remove X-Powered-By, which is an information leak
        fastcgi_hide_header X-Powered-By;

        # Path to the root of your installation
        root /var/www/html;

        # Specify how to handle directories -- specifying `/index.php$request_uri`
        # here as the fallback means that Nginx always exhibits the desired behaviour
        # when a client requests a path that corresponds to a directory that exists
        # on the server. In particular, if that directory contains an index.php file,
        # that file is correctly served; if it doesn't, then the request is passed to
        # the front-end controller. This consistent behaviour means that we don't need
        # to specify custom rules for certain paths (e.g. images and other assets,
        # `/updater`, `/ocm-provider`, `/ocs-provider`), and thus
        # `try_files $uri $uri/ /index.php$request_uri`
        # always provides the desired behaviour.
        index index.php index.html /index.php$request_uri;

        # Rule borrowed from `.htaccess` to handle Microsoft DAV clients
        location = / {
            if ( $http_user_agent ~ ^DavClnt ) {
                return 302 /remote.php/webdav/$is_args$args;
            }
        }

        location = /robots.txt {
            allow all;
            log_not_found off;
            access_log off;
        }

        # Make a regex exception for `/.well-known` so that clients can still
        # access it despite the existence of the regex rule
        # `location ~ /(\.|autotest|...)` which would otherwise handle requests
        # for `/.well-known`.
        location ^~ /.well-known {
            # The rules in this block are an adaptation of the rules
            # in `.htaccess` that concern `/.well-known`.

            location = /.well-known/carddav { return 301 /remote.php/dav/; }
            location = /.well-known/caldav  { return 301 /remote.php/dav/; }

            location /.well-known/acme-challenge    { try_files $uri $uri/ =404; }
            location /.well-known/pki-validation    { try_files $uri $uri/ =404; }

            # Let Nextcloud's API for `/.well-known` URIs handle all other
            # requests by passing them to the front-end controller.
            return 301 /index.php$request_uri;
        }

        # Rules borrowed from `.htaccess` to hide certain paths from clients
        location ~ ^/(?:build|tests|config|lib|3rdparty|templates|data)(?:$|/)  { return 404; }
        location ~ ^/(?:\.|autotest|occ|issue|indie|db_|console)                { return 404; }

        # Ensure this block, which passes PHP files to the PHP process, is above the blocks
        # which handle static assets (as seen below). If this block is not declared first,
        # then Nginx will encounter an infinite rewriting loop when it prepends `/index.php`
        # to the URI, resulting in a HTTP 500 error response.
        location ~ \.php(?:$|/) {
            # Required for legacy support
            rewrite ^/(?!index|remote|public|cron|core\/ajax\/update|status|ocs\/v[12]|updater\/.+|oc[ms]-provider\/.+|.+\/richdocumentscode\/proxy) /index.php$request_uri;

            fastcgi_split_path_info ^(.+?\.php)(/.*)$;
            set $path_info $fastcgi_path_info;

            try_files $fastcgi_script_name =404;

            include fastcgi_params;
            fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
            fastcgi_param PATH_INFO $path_info;
            #fastcgi_param HTTPS on;

            fastcgi_param modHeadersAvailable true;         # Avoid sending the security headers twice
            fastcgi_param front_controller_active true;     # Enable pretty urls
            fastcgi_pass php-handler;

            fastcgi_intercept_errors on;
            fastcgi_request_buffering off;
        }

        location ~ \.(?:css|js|svg|gif)$ {
            try_files $uri /index.php$request_uri;
            expires 6M;         # Cache-Control policy borrowed from `.htaccess`
            access_log off;     # Optional: Don't log access to assets
        }

        location ~ \.woff2?$ {
            try_files $uri /index.php$request_uri;
            expires 7d;         # Cache-Control policy borrowed from `.htaccess`
            access_log off;     # Optional: Don't log access to assets
        }

        # Rule borrowed from `.htaccess`
        location /remote {
            return 301 /remote.php$request_uri;
        }

        location / {
            try_files $uri $uri/ /index.php$request_uri;
        }
    }
}

Cheers for any insight.