Update installation.md to include multiple mounts and sample docker compose

[leewc]

Took awhile to figure out from https://www.reddit.com/r/selfhosted/comments/vxo7ga/filebrowser_multiple_directories/ and general searching.

Decided to raise a PR for it.

For the compose example, I used traefik, I had a bit more labels but not sure if repo owner wants to include traefik as example.

Traefik with labels + watchtower for upgrades.

services:
  filebrowser:
    image: filebrowser/filebrowser:s6
    container_name: filebrowser
    restart: unless-stopped
    volumes:
      - /docs:/srv/docs
      - /my-photos-videos:/srv/my-photos-videos
      - ./filebrowser:/database/
      - ./filebrowser:/config/
    environment:
      - PUID=1000
      - PGID=1000
    networks:
      - web
    labels:
      - 'traefik.enable=true'
      - 'traefik.http.routers.filebrowser.rule=Host(`files.example.com`)'
      - 'traefik.http.routers.filebrowser.entrypoints=https'
      - 'traefik.http.routers.filebrowser.tls.certResolver=primary'
      - 'traefik.http.routers.filebrowser.service=filebrowser'
      - 'traefik.http.services.filebrowser.loadBalancer.server.port=80'
      - 'traefik.docker.network=web'
      - 'com.centurylinklabs.watchtower.enable=true'

# Connect to pre-defined network so other containers with docker-compose can be discovered by Traefik
networks:
  web:
    external: true

Thank you for this awesome OSS project! Best one I have tried for self hosting thus far.