💚 : use mongodb 4.0 version

version 4.2 removes the $eval command which is used in our
MongoContainer.
see https://docs.mongodb.com/manual/release-notes/4.2-compatibility/#remove-support-for-the-eval-command

src/test/java/io/codeka/gaia/test/MongoContainer.java

@@ -27,7 +27,7 @@ public class MongoContainer extends GenericContainer {
     private MongoDatabase database;
 
     public MongoContainer() {
-        super("mongo");
+        super("mongo:4.0");
         setExposedPorts(List.of(MONGO_PORT));
     }