Fix beszel-template: use Docker volume instead of relative path

stacks/beszel-template.compose.yml

@@ -6,7 +6,7 @@ services:
     network_mode: host
     volumes:
       - /var/run/docker.sock:/var/run/docker.sock:ro
-      - ./beszel_agent_data:/var/lib/beszel-agent
+      - beszel_agent_data:/var/lib/beszel-agent
       # monitor other disks / partitions by mounting a folder in /extra-filesystems
       # - /mnt/disk/.beszel:/extra-filesystems/sda1:ro
     environment:
@@ -15,3 +15,5 @@ services:
       TOKEN: cfda0509-e9ed-4f56-80ff-0584976ef4cd
       HUB_URL: http://beszel.internal
 
+volumes:
+  beszel_agent_data: