fix webdav: enable VFS cache for MKCOL support

Zotero requires MKCOL (directory creation) which rclone's S3 backend
doesn't support without --vfs-cache-mode=full. Added tmpfs for the
cache directory.

compose.yml

@@ -292,6 +292,9 @@ services:
       - --addr=:8080
       - --user=${WEBDAV_USER:-zotero}
       - --pass=${WEBDAV_PASS}
+      - --vfs-cache-mode=full
+    tmpfs:
+      - /tmp/rclone-cache
     restart: unless-stopped
     depends_on:
       - rustfs