No but now you have a volume mount. Next step is to make sure uid 1000, as syncthing runs under by default, has write permission on the directory you created.
Just in case you weren’t already aware, an OCI container is an isolated environment, so the users inside the container don’t map 1:1 to users on the host OS, i.e., a user named “hostuser” with UID 1000 isn’t the same as a user named “containeruser” that happens to also be assigned UID 1000.
Search this forum for “portainer”, “docker” and “raspberry pi”. There are quite a few earlier posts that might be of help.
(Just my personal $0.02… If the goal of using Portainer and Docker is to learn about containers, that’s great. But if it’s simply to run Syncthing, it’s way overkill and an overly complex setup for a standalone application such as Syncthing that has minimal OS dependencies. Consider following the instructions on https://apt.syncthing.net/ instead.)
Search for “portainer”, “docker” and “raspberry pi” separately and it’ll turn up a bunch of earlier posts about working with Syncthing in Docker and also a Raspberry Pi (although not necessarily in combination).
Do the commands umount /media/usbhdd and mount /media/usbhdd return any errors? (The /etc/fstab entry shown above shouldn’t work because the syntax isn’t correct.)
ExFAT doesn’t support Unix file attributes such as ownership, so any chown after a volume is mounted is temporary (i.e. it only lasts as long as the volume is still mounted).