Bookstack behind letsencrypt, subfolder

Hello.
I am using letsencrypt docker, successfully set up as a reverse proxy with subdirectory configs. Everything is working good, exept that I have problems with bookstack behind the letsencrypt reverse proxy. It shows only the site without the css (?)
Here is a picture:


Here is my docker-compose file:

  bookstack:
    image: linuxserver/bookstack
    container_name: bookstack
    environment:
      - PUID=1000
      - PGID=1001
      - DB_HOST=bookstack_db
      - DB_USER=bookstack
      - DB_PASS=<password>
      - DB_DATABASE=bookstackapp
      - APP_URL=https://subdomain.domain.com/bookstack
    volumes:
      - /tank/docker/bookstack:/config
    ports:
      - 6875:80
    restart: unless-stopped
    depends_on:
      - bookstack_db
  bookstack_db:
    image: linuxserver/mariadb
    container_name: bookstack_db
    environment:
      - PUID=1000
      - PGID=1001
      - MYSQL_ROOT_PASSWORD=<password>
      - TZ=Europe/Oslo
      - MYSQL_DATABASE=bookstackapp
      - MYSQL_USER=bookstack
      - MYSQL_PASSWORD=<password>
    volumes:
      - /tank/docker/bookstack:/config
    restart: unless-stopped

I have all the services I have hostet at home on my subdomain, and every docker container behind a subdirectory.

Since letsencrypt docker comtainer doesn’t come with a bookstack proxy config, I wrote one myself. I’m not very good at nginx proxying, but used another service as a template. Here it is:

fyksen@fyksen-desktop:/tank/docker$ cat nginx/nginx/proxy-confs/bookstack.subfolder.conf
# set the CONTEXT_PATH variable to /booksonic in airsonic container.

location ^~ /bookstack {
    # enable the next two lines for http auth
    #auth_basic "Restricted";
    #auth_basic_user_file /config/nginx/.htpasswd;

    # enable the next two lines for ldap auth, also customize and enable ldap.conf in the default conf
    #auth_request /auth;
    #error_page 401 =200 /login;

    include /config/nginx/proxy.conf;
    resolver 127.0.0.11 valid=30s;
    set $upstream_bookstack bookstack;
    proxy_pass http://$upstream_bookstack:80;
}

From LAN, the service works correctly on hostip:6875

have you changed the .env file which should be in var/www/bookstack/ to add the APP_URL parameter?

Thanks for the reply.
Yeah, I have tried setting the .env app_url aswell.
I have tried with both setting the app_url in docker-compose and in .env.
Only in .env, and only in docker-compose.

Nothing seems to change the behaviour.