Docker - Traefik - Nextcloud - OnlyOffice: installed on a single host using multiple domains for the same Nextcloud container

docker-compose.yml

version: "3.3"

networks:
  proxy:
    ipam:
      driver: default
      config:
        - subnet: 17.100.0.0/16

services:

  traefik:
    image: traefik:v2.5
    container_name: traefik
    hostname: traefik
    networks:
      default:
      proxy:
        ipv4_address: 17.100.0.2
    command:
      #- "--log.level=DEBUG"
      #- "--certificatesresolvers.le.acme.caserver=https://acme-staging-v02.api.letsencrypt.org/directory"
      - "--api.insecure=true"
      - "--providers.docker=true"
      - "--providers.docker.exposedbydefault=false"


      - "--entrypoints.http.address=:80"
      - "--entrypoints.https.address=:443"
      - "--entrypoints.https.http.tls.certResolver=le"
      - "--entrypoints.http.http.redirections.entryPoint.to=entrypoints-https"
      - "--entrypoints.http.http.redirections.entryPoint.scheme=https"

      - "--certificatesResolvers.le.acme.email=email@address.com"
      - "--certificatesresolvers.le.acme.storage=/letsencrypt/acme.json"
      - "--certificatesResolvers.le.acme.tlsChallenge=true"
      - "--certificatesResolvers.le.acme.httpChallenge=true"
      - "--certificatesResolvers.le.acme.httpChallenge.entryPoint=http"
    volumes:
      - /docker/letsencrypt:/letsencrypt
      - /var/run/docker.sock:/var/run/docker.sock:ro
    ports:
      - 80:80
      - 443:443
      - 8080:8080
    restart: always

  mariadb:
    image: jc21/mariadb-aria:10.4
    container_name: mariadb
    hostname: mariadb
    environment:
      MYSQL_ROOT_PASSWORD: 'XXXX'
      MYSQL_DATABASE: 'nextcloud'
      MYSQL_USER: 'nextcloud'
      MYSQL_PASSWORD: 'XXXX'
    ports:
      - 3307:3306
    volumes:
      - /docker/mariadb:/var/lib/mysql
    restart: unless-stopped


  nextcloud:
    image: nextcloud:latest
    container_name: nextcloud
    hostname: nextcloud
    networks:
      default:
      proxy:
        ipv4_address: 17.100.0.3
    extra_hosts:
#       Add entry to /etc/hosts in container to Traefik proxy for OnlyOffice domain
      - "onlyoffice.domain.com:17.100.0.2"
    labels:

      - traefik.enable=true

      - traefik.http.routers.nextcloud.rule=Host(`nextcloud1.domain.com`, `nextcloud2.domain.com`)
      - traefik.http.routers.nextcloud.tls=true
      - traefik.http.routers.nextcloud.tls.certresolver=le

      - traefik.http.routers.nextcloud.middlewares=nextcloud,nextcloud_redirect

      - traefik.http.middlewares.nextcloud.headers.stsSeconds=155520011
      - traefik.http.middlewares.nextcloud.headers.stsIncludeSubdomains=true
      - traefik.http.middlewares.nextcloud.headers.stsPreload=true

      - traefik.http.middlewares.nextcloud.headers.accesscontrolalloworiginlist=*
      - traefik.http.middlewares.nextcloud.headers.customresponseheaders.X-Frame-Options=SAMEORIGIN

      - traefik.http.middlewares.nextcloud_redirect.redirectregex.permanent=true
      - traefik.http.middlewares.nextcloud_redirect.redirectregex.regex=/.well-known/(card|cal)dav
      - traefik.http.middlewares.nextcloud_redirect.redirectregex.replacement=/remote.php/dav/

    ports:
      - 8888:80
    volumes:
      - /docker/nextcloud:/var/www/html
      - /mnt/time-machine/nextcloud:/var/www/html/data
      - /mnt:/mnt
    restart: unless-stopped


  onlyoffice:
    image: alehoho/oo-ce-docker-license
    container_name: onlyoffice
    hostname: onlyoffice
    networks:
      default:
      proxy:
        ipv4_address: 17.100.0.4
    extra_hosts:
#       Add entry to /etc/hosts in container to Traefik proxy for OnlyOffice and NextCloud domains
      - "onlyoffice.domain.com:17.100.0.2"
      - "nextcloud1.domain.com:17.100.0.2"
      - "nextcloud2.domain.com:17.100.0.2"
    labels:
#       https://helpcenter.onlyoffice.com/de/installation/docs-community-proxy.aspx

      - traefik.enable=true

      - traefik.http.routers.onlyoffice.rule=Host(`onlyoffice.domain.com`)
      - traefik.http.routers.onlyoffice.tls=true
      - traefik.http.routers.onlyoffice.tls.certresolver=le

      - traefik.http.routers.onlyoffice.middlewares=onlyoffice

      - traefik.http.middlewares.onlyoffice.headers.customrequestheaders.X-Forwarded-Host=onlyoffice.domain.com
      - traefik.http.middlewares.onlyoffice.headers.customrequestheaders.X-Forwarded-Proto=https

    ports:
      - 9981:80
    restart: unless-stopped

Docker uses it for the virtual network between containers. Not sure if it routes it externally or not. Regardless... what I have here seems to do the trick.

docker uses 172.17.0.0/16 by default, if you are not using swarm. (and every other private address space counting upwards)
It uses 17.100.0.0/16 because you told it to do so.

As long as you don't serve to people whose ISP have 17.100 address space...