diff --git a/README.md b/README.md
index 7e2f045..da13c71 100644
--- a/README.md
+++ b/README.md
@@ -158,7 +158,7 @@ Environment variables:
 |----------|-------------|---------|
 | `PORT` | HTTP listen port | 8080 |
 | `UPAAS_DATA_DIR` | Data directory for SQLite and keys | ./data |
-| `UPAAS_HOST_DATA_DIR` | Host path for DATA_DIR (when running in container) | same as DATA_DIR |
+| `UPAAS_HOST_DATA_DIR` | Host path for DATA_DIR (when running in container) | *(none — must be set to an absolute path)* |
 | `UPAAS_DOCKER_HOST` | Docker socket path | unix:///var/run/docker.sock |
 | `DEBUG` | Enable debug logging | false |
 | `SENTRY_DSN` | Sentry error reporting DSN | "" |
@@ -187,9 +187,9 @@ services:
       - "8080:8080"
     volumes:
       - /var/run/docker.sock:/var/run/docker.sock
-      - ${HOST_DATA_DIR:-./data}:/var/lib/upaas
+      - ${HOST_DATA_DIR}:/var/lib/upaas
     environment:
-      - UPAAS_HOST_DATA_DIR=${HOST_DATA_DIR:-./data}
+      - UPAAS_HOST_DATA_DIR=${HOST_DATA_DIR}
       # Optional: uncomment to enable debug logging
       # - DEBUG=true
       # Optional: Sentry error reporting