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.yml

@@ -53,10 +53,12 @@ services:
       POSTGRES_HOST: postgres
       POSTGRES_PORT: 5432
       REDIS_URL: redis://redis:6379/0
-      JWT_SECRET: ${JWT_SECRET:-change-me-in-production}
+      SESSION_SECRET: ${SESSION_SECRET:-change-me-in-production}
       REPO_BASE_PATH: /data/repos
+      INSTANCE_BASE_PATH: /data/instances
     volumes:
       - repo_data:/data/repos
+      - instance_data:/data/instances
     ports:
       - "8000:8000"
     depends_on:
@@ -90,6 +92,7 @@ volumes:
   postgres_data:
   redis_data:
   repo_data:
+  instance_data:
 
 networks:
   backend: