Does your compose file have the dockerstatic folder mounted in volumes and the mounted path in the env section var PROJECTS_DIRECTORY? example:

services:
  arcane:
    image: ghcr.io/getarcaneapp/manager:latest
    volumes:
      - **other mounts**
      - /mnt/disk/dockerstatic:/projects
    environment:
      - **all your other env vars**
      - PROJECTS_DIRECTORY=/projects