nginx configuration for nextcloud with path

I run nextcloud on docker (image nextcloud:production-fpm) with an nginx-container (image: nginx) as reverse-proxy (handling SSL-termination) based on the example nginx-configuation (https://raw.githubusercontent.com/nextcloud/docker/master/.examples/docker-compose/insecure/mariadb/fpm/web/nginx.conf). This works.

Now however I want nextcloud to be served in an path (i.e. mydomain.com/nextcloud instead of mydomain). I found an github-issue on this (https://github.com/nextcloud/docker/issues/401) which point at the nginx-configuration, but this only solves it for very easy configurations (adapting the nextcloud by adding - OVERWRITEWEBROOT=/nextcloud to the environment is the easiest part). Does anybody now how to adapt the nginx-configuration to serve nextcloud from a path?

I created a demo-project to illustrate my problem: compose.yml:

version: '3'

volumes:
  nextcloud:
  mariadb:

services:
  mariadb:
    image: mariadb
    container_name: mariadb-subdir
    command: --transaction-isolation=READ-COMMITTED --binlog-format=ROW
    volumes:
      - mariadb:/var/lib/mysql
    environment:
      - MYSQL_ROOT_PASSWORD_FILE=/run/secrets/mariadb_root_password
      - MYSQL_PASSWORD_FILE=/run/secrets/mariadb_password
      - MYSQL_DATABASE_FILE=/run/secrets/mariadb_db
      - MYSQL_USER_FILE=/run/secrets/mariadb_user
    secrets:
      - mariadb_root_password
      - mariadb_password
      - mariadb_user
      - mariadb_db

  nextcloud:
    image: nextcloud:production-fpm
    container_name: nextcloud-subdir
    links:
      - mariadb
    volumes:
      - nextcloud:/var/www/html
    environment:
      - MYSQL_PASSWORD_FILE=/run/secrets/mariadb_password
      - MYSQL_DATABASE_FILE=/run/secrets/mariadb_db
      - MYSQL_USER_FILE=/run/secrets/mariadb_user
      - MYSQL_HOST=mariadb
      - NEXTCLOUD_ADMIN_PASSWORD_FILE=/run/secrets/nextcloud_admin_password
      - NEXTCLOUD_ADMIN_USER_FILE=/run/secrets/nextcloud_admin_user
      - NEXTCLOUD_TRUSTED_DOMAINS=localhost
      # (un)comment to switch nextcloud being served in "/nextcloud" (with adapted proxy configuration)
      - OVERWRITEWEBROOT=/nextcloud
    secrets:
      - mariadb_password
      - mariadb_user
      - mariadb_db
      - nextcloud_admin_password
      - nextcloud_admin_user

  web:
    image: nginx
    container_name: nginx-subdir
    ports:
      - 80:80
    links:
      - nextcloud
    volumes:
      # (un)comment to switch nextcloud being served in "/nextcloud" (with adapted OVERWRITEWEBROOT variable)
      #- ./nginx.conf:/etc/nginx/nginx.conf:ro
      - ./nginx_subdir.conf:/etc/nginx/nginx.conf:ro
    volumes_from:
      - nextcloud
      
secrets:
    # files are supposed to contain only usernames or passwords and a newline
    nextcloud_admin_password:
      file: ./nextcloud_admin_password.txt
    nextcloud_admin_user:
      file: ./nextcloud_admin_user.txt
    mariadb_root_password:
      file: ./mariadb_root_password.txt
    mariadb_db:
      file: ./mariadb_db.txt
    mariadb_user:
      file: ./mariadb_user.txt
    mariadb_password:
      file: ./mariadb_passwort.txt

The nginx.conf is https://raw.githubusercontent.com/nextcloud/docker/master/.examples/docker-compose/insecure/mariadb/fpm/web/nginx.conf the user.txt and password.txt files are simple textfiles containing the values for initialization.

The nginx_subdir.conf:

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 nextcloud: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 = /nextcloud/ {
            if ( $http_user_agent ~ ^DavClnt ) {
                add_header debuglocation DavClnt always;
                return 302 /nextcloud/remote.php/webdav/$is_args$args;
            }
        }

        location = /nextcloud/robots.txt {
            add_header debuglocation robots always;
            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 ^~ nextcloud/.well-known {
            add_header debuglocation well-known always;

            # The rules in this block are an adaptation of the rules
            # in `.htaccess` that concern `/.well-known`.
            rewrite ^/localhost/nextcloud(.*) $1 break;

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

            location nextcloud/.well-known/acme-challenge    { try_files $uri $uri/ =404; }
            location nextcloud/.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 nextcloud/index.php$request_uri;
        }

        # Rules borrowed from `.htaccess` to hide certain paths from clients
        location ~ ^/nextcloud/(?:build|tests|config|lib|3rdparty|templates|data)(?:$|/)  {
            add_header debuglocation internal_1 always;
            return 404;
        }
        location ~ ^/nextcloud/(?:\.|autotest|occ|issue|indie|db_|console)                {
            add_header debuglocation internal_2 always;
            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(?:$|/) {
            add_header debuglocation PHP always;

            rewrite ^/localhost/nextcloud(.*) $1;
            # Required for legacy support
            rewrite ^/(?!index|remote|public|cron|core\/ajax\/update|status|ocs\/v[12]|updater\/.+|oc[ms]-provider\/.+|.+\/richdocumentscode\/proxy) /index.php$request_uri;

            fastcgi_split_path_info ^(.+?\.php)(/.*)$;
            set $path_info $fastcgi_path_info;
            add_header debuglocation2 $path_info always;

            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)$ {
            add_header debuglocation images always;
            rewrite ^/localhost/nextcloud(.*) $1 break;
            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?$ {
            add_header debuglocation woff2 always;
            rewrite ^/localhost/nextcloud(.*) $1 break;
            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 /nextcloud/remote {
            add_header debuglocation remote always;
            return 301 /nextcloud/remote.php$request_uri;
        }

        location /nextcloud/ {
            add_header debuglocation nextcloud always;
            rewrite ^/localhost/nextcloud(.*) $1 break;
            try_files $uri $uri/ /index.php$request_uri;
        }
    }
}

In this I tried to add the rewrite rule rewrite ^/localhost/nextcloud(.*) $1 break; to all relevant locations and added add_header debuglocation to all locations for debugging them with curl -v (thanks to https://stackoverflow.com/questions/12703702/nginx-test-which-location-used-to-process-request). Calling curl -v http://localhost/nextcloud/ > /dev/null returns a 404 but without debuglocation populated.