I found the solution to this issue on this page: https://github.com/cristianofromagio/laravel-docker
The cache must be cleared, which I performed on Synology Docker by running:
docker exec -it “ContainerName” php /var/www/localhost/heimdall/artisan config:cache
I then restarted the container, and the error went away. [Note that you cannot run this command if the container is not yet running.]
(Incidentally, I then found another permissions issue, which I will report separately.)