fix: mount /data/instances as host bind mount for API

When API runs in Docker with named volume instance_data:/data/instances,
generated docker-compose.yml files use bind mounts like
/data/instances/.../repo-clone:/workspace. Docker resolves bind mounts
on the host filesystem, not in named volumes, so containers see empty
 directories.

By mounting /data/instances as a host bind mount, both the API and
generated tool containers access the same host path.

.stoneforge/.dashboard-opened

@@ -1,2 +1,2 @@
-84102
-1779616938758
+130078
+1779618546909

docker-compose.yml

@@ -58,7 +58,7 @@ services:
       INSTANCE_BASE_PATH: /data/instances
     volumes:
       - repo_data:/data/repos
-      - instance_data:/data/instances
+      - /data/instances:/data/instances
     ports:
       - "8000:8000"
     depends_on:
@@ -92,7 +92,6 @@ volumes:
   postgres_data:
   redis_data:
   repo_data:
-  instance_data:
 
 networks:
   backend: