Make sure the solr volume has an explicit name

This only matters for docker-compose v1,
but without the name the volume won't be deleted
when the project is deleted.

Author: rfay

docker-compose.solr.yaml

@@ -81,6 +81,5 @@ services:
 
 volumes:
   # solr is a persistent Docker volume for solr data
-  # The persistent volume should have the same name as the service so it can be deleted
-  # when the project is deleted.
   solr:
+    name: "ddev-${DDEV_SITENAME}_solr"