make a named volume for the Bety logs mountpoint

Best I can tell nothing gets written to this, but because the Bety image
specifies this path as a VOLUME, we were creating a new anonymous-but-permanent
volume every time Compose spun up

docker-compose.yml

@@ -79,6 +79,8 @@ services:
       - SECRET_KEY_BASE=${BETY_SECRET_KEY:-notasecret}
       - RAILS_RELATIVE_URL_ROOT=/bety
       - LOCAL_SERVER=${BETY_LOCAL_SERVER:-99}
+    volumes:
+      - bety:/home/bety/log
     depends_on:
       - postgres
     labels:
@@ -346,6 +348,7 @@ networks:
 volumes:
   traefik:
   postgres:
+  bety:
   rabbitmq:
   pecan:
   rstudio: