Added missing immutable config flag to Compose file

The cron service for Docker Compose was previously missing the AUR_CONFIG_IMMUTABLE environment variable, which causes issues when a different database name/user/password is used from the default due to the aurweb-config calls at https://gitlab.archlinux.org/archlinux/aurweb/-/blob/master/docker/cron-entrypoint.sh#L5.

There also appears to be some other services like fastapi that are missing this variable - should I also add the immutable switch to those services in this MR?