remove outdated env vars and fix make_last_user_admin

abra.sh

@@ -1,6 +1,8 @@
 export LOOMIO_ENTRYPOINT_VERSION=v5
 
+# cannot be integrated into entrypoint.sh as it requires the operator to create a user first
 function make_last_user_admin()
 {
+    export DATABASE_URL="postgresql://${POSTGRES_USER}:$(cat /run/secrets/db_password)@db/${POSTGRES_DB}"
     SECRET_KEY_BASE=$(rake secret) rails runner "User.last.update(is_admin: true)"
 }