ports in docker-compose.yml must be an array

docker-compose.yml

@@ -16,7 +16,8 @@ wekandb:
 #    - ./data/runtime/db:/data/db
 #    - ./data/dump:/dump
   command: mongod --smallfiles --oplogSize 128
-  ports: 27017
+  ports:
+    - 27017
 
 wekan:
   image: mquandalle/wekan