jwilder/nginx-proxy setup with multiple php fpm containers

Question

I am trying to run the official nextcloud docker-compose.yml(https://github.com/nextcloud/docker/blob/master/.examples/docker-compose.yml) with the mailcow-dockerized(https://github.com/mailcow/mailcow-dockerized) project on one vps.
It actually works, I can access both applications in the browser(even email sending and receiving works) :), there is only one drawback, as soon as both apps try to access fpm, the vps crashes.
So I was thinking this probably has to do that they both want to access port 9000 of the host.

So the next thing I tried to do, was changing the per host configuration to add fastcgi pass:
.examples/proxy/VIRTUAL_HOST_location

fastcgi_pass nextcloud_fpm:9000;

But I get a blank page with no nginx or fastcgi errors(docker logs proxy -f --since 2). And docker logs nextcloud_fpm -f --since 2 outputs the following:

172.19.0.7 - 08/Sep/2017:13:07:32 +0000 "- " 200
172.19.0.7 - 08/Sep/2017:13:07:40 +0000 "- " 200

version: '2' services:   proxy:
    image: jwilder/nginx-proxy
    container_name: proxy
    ports:
      - 80:80
      - 443:443
    volumes:
      - ./proxy/conf.d:/etc/nginx/conf.d
      - ./proxy/vhost.d:/etc/nginx/vhost.d
      - ./proxy/html:/usr/share/nginx/html
      - ./proxy/certs:/etc/nginx/certs:ro
      - /var/run/docker.sock:/tmp/docker.sock:ro
    networks:
      - proxy-tier
    restart: always

  letsencrypt-companion:
    image: alastaircoote/docker-letsencrypt-nginx-proxy-companion
    container_name: letsencrypt-companion
    volumes_from:
      - proxy
    volumes:
      - /var/run/docker.sock:/var/run/docker.sock:ro
      - ./proxy/certs:/etc/nginx/certs:rw
    restart: always

  web:
    image: nginx
    container_name: nextcloud_webserver
    volumes:
      - ./nginx.conf:/etc/nginx/nginx.conf:ro
    links:
      - app
      - collabora
    volumes_from:
      - app
    environment:
      - VIRTUAL_HOST=${DOMAIN}
      - VIRTUAL_NETWORK=nginx-proxy
      - VIRTUAL_PORT=80
      - LETSENCRYPT_HOST=${DOMAIN}
      - LETSENCRYPT_EMAIL=
    networks:
      - proxy-tier
    restart: always

  app:
    image: nextcloud:fpm
    container_name: nextcloud_fpm
    links:
      - db
    volumes:
      - ./nextcloud/apps:/var/www/html/apps
      - ./nextcloud/config:/var/www/html/config
      - ./nextcloud/data:/var/www/html/data
    networks:
      - proxy-tier
    restart: always

  db:
    image: mariadb
    container_name: db
    volumes:
      - ./nextcloud/db:/var/lib/mysql
    environment:
      - MYSQL_ROOT_PASSWORD=
      - MYSQL_DATABASE=nextcloud
      - MYSQL_USER=nextcloud
      - MYSQL_PASSWORD=
    networks:
      - proxy-tier
    restart: always

  redis:
    image: redis
    container_name: redis
    networks:
      - proxy-tier
    restart: always

  collabora:
    image: collabora/code
    container_name: collabora
    cap_add:
      - MKNOD
    environment:
      - domain=${DOMAIN}
    networks:
      - proxy-tier
    restart: always

networks:   proxy-tier:
    external:
      name: nginx-proxy

Excerpt of the mailcow docker-compose(with the changed part from the original):

  php-fpm-mailcow:
      image: mailcow/phpfpm:1.0
      build: ./data/Dockerfiles/phpfpm
      command: "php-fpm -d date.timezone=${TZ}"
      depends_on:
        - redis-mailcow
      volumes:
        - ./data/web:/web:ro
        - ./data/conf/rspamd/dynmaps:/dynmaps:ro
        - dkim-vol-1:/data/dkim
      environment:
        - DBNAME=${DBNAME}
        - DBUSER=${DBUSER}
        - DBPASS=${DBPASS}
        - MAILCOW_HOSTNAME=${MAILCOW_HOSTNAME}
        - IMAP_PORT=${IMAP_PORT:-143}
        - IMAPS_PORT=${IMAPS_PORT:-993}
        - POP_PORT=${POP_PORT:-110}
        - POPS_PORT=${POPS_PORT:-995}
        - SIEVE_PORT=${SIEVE_PORT:-4190}
        - SUBMISSION_PORT=${SUBMISSION_PORT:-587}
        - SMTPS_PORT=${SMTPS_PORT:-465}
        - SMTP_PORT=${SMTP_PORT:-25}
      restart: always
      dns:
        - 172.22.1.254
      dns_search: mailcow-network
      networks:
        mailcow-network:
          aliases:
            - phpfpm

  nginx-mailcow:
      depends_on:
        - sogo-mailcow
        - php-fpm-mailcow
      image: nginx:mainline-alpine
      healthcheck:
        test: ["CMD", "ping", "php-fpm-mailcow", "-c", "5"]
        interval: 5s
        timeout: 5s   
        retries: 10
      command: /bin/sh -c "envsubst < /etc/nginx/conf.d/templates/listen_plain.template > /etc/nginx/conf.d/listen_plain.active &&
        envsubst < /etc/nginx/conf.d/templates/listen_ssl.template > /etc/nginx/conf.d/listen_ssl.active &&
        envsubst < /etc/nginx/conf.d/templates/server_name.template > /etc/nginx/conf.d/server_name.active &&
        nginx -g 'daemon off;'"
      environment:
        - HTTPS_PORT=${HTTPS_PORT:-443}
        - HTTP_PORT=${HTTP_PORT:-80}
        - MAILCOW_HOSTNAME=${MAILCOW_HOSTNAME}
        - VIRTUAL_HOST=${MAILCOW_HOSTNAME}
        - VIRTUAL_NETWORK=nginx-proxy
        - VIRTUAL_PORT=${HTTP_PORT:-80}
        - LETSENCRYPT_HOST=${MAILCOW_HOSTNAME}
        - LETSENCRYPT_EMAIL=${LETSENCRYPT_EMAIL}
          volumes:
            - ./data/web:/web:ro
            - ./data/conf/rspamd/dynmaps:/dynmaps:ro
            #- ./data/assets/ssl/:/etc/ssl/mail/:ro
            - mailcerts:/etc/ssl/mail/:ro
            - ./data/conf/nginx/:/etc/nginx/conf.d/:rw
          expose:
            - "${HTTP_PORT:-80}"
          #ports:
          #  - "${HTTPS_BIND:-0.0.0.0}:${HTTPS_PORT:-443}:${HTTPS_PORT:-443}"
          #  - "${HTTP_BIND:-0.0.0.0}:${HTTP_PORT:-80}:${HTTP_PORT:-80}"
          restart: always
          dns:
            - 172.622.1.254
          dns_search: mailcow-network
          networks:
            mailcow-network:
              ipv4_address: 172.22.1.251
              aliases:
                - nginx
            proxy-tier:

# At the end of the docker-compse:
networks:
  mailcow-network:
    driver: bridge
    enable_ipv6: true
    ipam:
      driver: default
      config:
        - subnet: 172.22.1.0/24
        - subnet: fd4d:6169:6c63:6f77::/64
  proxy-tier:
    external:
      name: nginx-proxy

volumes:
  vmail-vol-1:
  mysql-vol-1:
  dkim-vol-1:
  redis-vol-1:
  rspamd-vol-1:
  postfix-vol-1:
  crypt-vol-1:
  mailcerts:
    driver_opts:
      type: none
      device: /opt/nextcloud-docker/.examples/proxy/certs/${MAILCOW_HOSTNAME}
      o: bind

So I am seeing now, they have their individual nginx files. The part for nextcloud(also available on github repo, but I cannot post more than 2 links, thanks sf!):

user www-data;

events {
  worker_connections 768;
}

http {
  upstream backend {
      server app:9000;
  }
  include /etc/nginx/mime.types;
  default_type application/octet-stream;

  server {
    listen 80;

    # Add headers to serve security related headers
    add_header Strict-Transport-Security "max-age=15768000; includeSubDomains; preload;";
    add_header X-Content-Type-Options nosniff;
    add_header X-XSS-Protection "1; mode=block";
    add_header X-Robots-Tag none;
    add_header X-Download-Options noopen;
    add_header X-Permitted-Cross-Domain-Policies none;

    root /var/www/html;
    client_max_body_size 10G; # 0=unlimited - set max upload size
    fastcgi_buffers 64 4K;

    gzip off;

    index index.php;
    error_page 403 /core/templates/403.php;
    error_page 404 /core/templates/404.php;

    rewrite ^/.well-known/carddav /remote.php/dav/ permanent;
    rewrite ^/.well-known/caldav /remote.php/dav/ permanent;

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

    location ~ ^/(build|tests|config|lib|3rdparty|templates|data)/ {
      deny all;
    }

    location ~ ^/(?:\.|autotest|occ|issue|indie|db_|console) {
      deny all;
    }

    location / {
      rewrite ^/remote/(.*) /remote.php last;
      rewrite ^(/core/doc/[^\/]+/)$ $1/index.html;
      try_files $uri $uri/ =404;
    }

    location ~ \.php(?:$|/) {
      fastcgi_split_path_info ^(.+\.php)(/.+)$;
      include fastcgi_params;
      fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
      fastcgi_param PATH_INFO $fastcgi_path_info;
      fastcgi_param HTTPS on;
      fastcgi_param modHeadersAvailable true; #Avoid sending the security headers twice
      fastcgi_pass backend;
      fastcgi_intercept_errors on;
    }

    # Adding the cache control header for js and css files
    # Make sure it is BELOW the location ~ \.php(?:$|/) { block
    location ~* \.(?:css|js)$ {
      add_header Cache-Control "public, max-age=7200";
      # Add headers to serve security related headers
      add_header Strict-Transport-Security "max-age=15768000; includeSubDomains; preload;";
      add_header X-Content-Type-Options nosniff;
      add_header X-Frame-Options "SAMEORIGIN";
      add_header X-XSS-Protection "1; mode=block";
      add_header X-Robots-Tag none;
      add_header X-Download-Options noopen;
      add_header X-Permitted-Cross-Domain-Policies none;
      # Optional: Don't log access to assets
      access_log off;
    }

    # Optional: Don't log access to other assets
    location ~* \.(?:jpg|jpeg|gif|bmp|ico|png|swf)$ {
      access_log off;
    }

    # collabora static files
    location ^~ /loleaflet {
        proxy_pass https://collabora:9980;
        proxy_set_header Host $http_host;
    }

    # collabora WOPI discovery URL
    location ^~ /hosting/discovery {
        proxy_pass https://collabora:9980;
        proxy_set_header Host $http_host;
    }

    # collabora websockets, download, presentation and image upload
    location ^~ /lool {
        proxy_pass https://collabora:9980;
        proxy_set_header Upgrade $http_upgrade;
        proxy_set_header Connection "upgrade";
        proxy_set_header Host $http_host;
    }
  }
}

That means, I already wrongly took the containername for the fastcgi pass, instead of the app container from the docker-compose :(.
But that results in *4 connect() failed (111: Connection refused) while connecting to upstream, client:

The part for mailcow(also available in github repo):

server {
  listen 8081;
  listen [::]:8081;
  index index.php index.html;
  server_name _;
  error_log  /var/log/nginx/error.log;
  access_log /var/log/nginx/access.log;
  root /dynmaps;

  location ~ \.php$ {
    try_files $uri =404;
    fastcgi_split_path_info ^(.+\.php)(/.+)$;
    fastcgi_pass phpfpm:9000;
    fastcgi_index index.php;
    include fastcgi_params;
    fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
    fastcgi_param PATH_INFO $fastcgi_path_info;
  }
}

I am unsure now, what makes my server crash. What I do know is docker ps | grep 9000 outputs this:

4c04c978c4d5        nextcloud:fpm                                            "/entrypoint.sh ph..."   8 minutes ago       Up 7 minutes          9000/tcp                                                                                                                                                        nextcloud_fpm
9ff7a19d0253        mailcow/phpfpm:1.0                                       "/docker-entrypoin..."   2 days ago          Up 2 days             9000/tcp                                                                                                                                                        mailcowdockerized_php-fpm-mailcow_1

And I still think, that makes it crash.

nmap -p 9000 cloud.mydomain.com outputs this:

Starting Nmap 7.01 at 2017-09-09 09:14 CEST
Nmap scan report for cloud.mydomain.com (ip)
Host is up (0.029s latency).
rDNS record for ip: mail.mydomain.com
PORT     STATE  SERVICE
9000/tcp closed cslistener

Nmap done: 1 IP address (1 host up) scanned in 0.10 seconds

I don't understand, what that means, using nmap for the first time.