The CDN where the files were hosted no longer exists. Now the hoster has to provide the files in order to run. These can be downloaded through SteamCMD or Steam.
Download via SteamCMD using Docker:
docker run --rm -it -v ./data:/data steamcmd/steamcmd:latest +@sSteamCmdForcePlatformType windows +force_install_dir /data +login <username> <password> +app_update 2915930 +quit
In the compose.yaml you now need to change the volume server-data to point to your location of the files.
The files should be mounted to /data inside the container.
Example:
/SomePathThatContainsTheSteamFiles # this needs to be mounted to /data in the container
KnockoutCity # can be removed for servers as its not needed
KnockoutCityServer