The problem is the smbclient packages need to be installed inside the Docker container. Installing it on the Docker host OS won’t work. I don’t know why it isn’t included. It would save a lot of people a lot of trouble if they included the two little packages.
You can open a shell in the container using docker exec, then install with apt. This is only a temporary fix because next time you pull an updated container image, it won’t have smbclient again.
A more permanent fix is to build the container image yourself. Docker-compose can do this mostly automatically once you set it up.
You can find out how to do this here:
https://github.com/docker-library/docs/blob/master/nextcloud/README.md#adding-features
I also cover it briefly in my guide:
https://help.nextcloud.com/t/howto-ubuntu-docker-nextcloud-talk-collabora/76430