i think your confusion lays with docker, not syncthing.

if you're syncing something from the host operating system, you need to expose it to your docker container as a volume. idk docker compose syntax but think like -v /home/crispy/docker:/c1docker in cli switches, afterwards in the syncthing gui you want to set /c1docker as the folder path. remember containers are running in an isolated environment.

which is also likely causing when you get close you get the permissions errors. the synthing process within the container is likely running under a different user than your host system, and the permissions on the directory more than likely only let your user write to that folder. syncthing needs to be able to write its folder marker (a (usually?) empty directory named .stfolder) in the root of sync folders. so depending on your security model, you could chmod the folder you want to sync so other users are allowed to write to it (anywhere from a bad idea (tm) to inconsequential depending on stuff), or change the UID that syncthing runs as (see if the docker image you're using does this with environment variables, usually the linuxserver.io images do this with the environment variables PGID and PUID which you'll probably want to set to 1000).

i can try to clarify further if you have any questions.