Or this?

Dockerfile

@@ -1,5 +1,5 @@
 ## mongo build stage
-ARG MONGO_VERSION=${MONGO_VERSION}
+#ARG MONGO_VERSION=${MONGO_VERSION}
 FROM mongo:${MONGO_VERSION:-latest} AS database
 WORKDIR /
 COPY mongo-init.js ./docker-entrypoint-initdb.d/

docker-compose.staging-build.yml

@@ -9,7 +9,7 @@ services:
             context: .
             target: database
             args:
-              - MONGO_VERSION
+              MONGO_VERSION: ${MONGO_VERSION}
         networks:
             - docknet
         volumes: