fix(docker): make instance directory configurable and writable

- Add INSTANCE_BASE_PATH config option (defaults to /data/instances)
- Update docker.py to use configured path instead of hardcoded 'data/instances'
- Update Dockerfile to create /data/instances and chown to appuser
- Add instance_data volume to docker-compose.traefik.yml and docker-compose.yml
- Set INSTANCE_BASE_PATH env var in both compose files

This fixes the PermissionError when creating tool instances because
appuser can now write to /data/instances.

docker-compose.traefik.yml

@@ -80,6 +80,7 @@ services:
       SESSION_SECRET: ${SESSION_SECRET:-change-me-in-production}
       SESSION_TTL_HOURS: ${SESSION_TTL_HOURS:-24}
       REPO_BASE_PATH: /data/repos
+      INSTANCE_BASE_PATH: /data/instances
       API_DOMAIN: ${API_DOMAIN}
       WEB_DOMAIN: ${WEB_DOMAIN}
       AUTHENTIK_DOMAIN: ${AUTHENTIK_DOMAIN}
@@ -92,6 +93,7 @@ services:
       AUTHENTIK_TOKEN_URL: ${AUTHENTIK_TOKEN_URL:-}
     volumes:
       - repo_data:/data/repos
+      - instance_data:/data/instances
       - avatar_uploads:/app/uploads
     depends_on:
       postgres:
@@ -114,6 +116,7 @@ volumes:
   postgres_data:
   redis_data:
   repo_data:
+  instance_data:
   avatar_uploads:
 
 networks: