How to set custom db_mem for Fulcrum to speed up initial sync

I'm running Fulcrum 2.1.0 on umbrelOS and trying to set db_mem = 4096 to speed up the initial sync. I edited the file at /home/umbrel/umbrel/app-data/fulcrum/data/fulcrum.conf and added the line but Fulcrum still logs DB memory: 2048 MiB after a restart. Running sudo docker inspect fulcrum_fulcrum_1 | grep fulcrum.conf returns nothing, suggesting the config file isn't being mounted by the container. How can I pass a custom db_mem value to Fulcrum on umbrelOS?