Lighttpd forwarding to docker containers help, please

Hello fellow Dietpiers.
I could use some help with reverse proxy to Docker containers, been driving me nuts for weeks. I picked up a new Nuc to step up from a Raspberry Pi 4. Fresh install with lighttpd,
nextcloud, owncloud, certbot, Docker, Docker-Compose and Portainer. Lighttpd is serving my website, NC and OC perfectly thru https, Android apps and PC programs. In docker I only want to run Bookstack, Vaultwarden and StandardNotes.
I started off with Bootstack, edited, enabled and reloaded lighttpd with this code

$HTTP["url"] =~ "^/bookstack($|/)" {
  proxy.header = (
    "map-urlpath" => (
      "/bookstack"  => ""
  proxy.server = ( "" =>
    (( "host" => "", "port" => 8080 ))

Ignore the port I adjusted in docker-compose, this got me nowhere. So I switched to the solidnerd version and got the login page. However after logging in I get ‘page not found’.
Looking at the browser address bar I noticed ‘
a double address of bookstack.
The container uses apache2 and the solidnerd uses nginx and both produce different results.
I am at a loss with what to do. Do I clean install ( no problem it takes less then 3 minutes with my Redo Rescue backup base) and try with nginx? I would hate to loose my excellent connectivity I get with lighttpd, or is it possible to sort this out? I should point out I like the sub-path vs sub-domain concept Dietpi uses and would rather stick with that instead of creating a bunch of subdomains.
Any help would be very much appreciated, sorry for such a long post.

For Bookstack you need to configure the docker container to be aware on the sub directory Bookstack should be reachable on. Did you set the -e APP_URL= value inside your compose configuration? Docker

Anyway, lighttpd isn’t that good web server to run as revers proxy. Nginx or Apache would be way better for this purpose.

First ensure your Bookstack is reachable on the sub directory http://your.local.ip.address/bookstack

Once done you could continue to play with lighttpd as proxy or think of using Nginx Proxy Manager (docker container).

The map-urlpath removes the path element from the request, as least it should as far as I underatand it. Just not sure whether in return it is added again e.g. for internal links/redirects etc. So probably indeed if Bookstack can deal with an URL path (can be configured to), it might be better to keep it, i.e. remove the map-urlpath block.

lighttpd "map-urlpath" maps the URL path in the HTTP request. "map-urlpath" does not rewrite links in the response body returned by bookstack. Perhaps you have to tell bookstack that bookstack is located under “/bookstack” so that it generates external links that start with “/bookstack”. On the server hosting bookstack, you may have to create a symlink in the document root from bookstack => . so that any URLs that reach the backend server resolve the “/bookstack” prefix of the URL path to the document root. “/bookstack/bookstack” would also resolve to the document root on the backend server.