Change to previous MongoDB version 3.2.18 because of data loss:

https://github.com/wekan/wekan-mongodb/issues/9

docker-compose.yml

@@ -3,7 +3,7 @@ version: '2'
 services:
 
   wekandb:
-    image: mongo:3.2.19
+    image: mongo:3.2.18
     container_name: wekan-db
     restart: always
     command: mongod --smallfiles --oplogSize 128