add directions on building apptainer.def

README.md

@@ -39,6 +39,13 @@ apptainer build --sandbox node/ apptainer.def
 apptainer run --writable node/ 
 ```
 
+An apptainer.def file can be built directory from the dockerfile as needed:
+
+```
+pip3 install spython
+spython recipe Dockerfile &> apptainer.def
+```
+
 ## API Docs
 
 See the [API documentation page](https://github.com/OpenDroneMap/NodeODM/blob/master/docs/index.adoc).