command not found: sudo systemctl restart shiny-server, same with start and stop

I have made an update to /etc/shiny-server/shiny-server.conf and wanted to ensure this update is 'active' by restarting shiny server.

Reading section 1.4 on stopping and starting, we are told the way to d this is with sudo systemctl restart shiny-server

However, this returns 'command not found' message in the terminal. Same with start and stop.

I'm running rocker/tidyverse Docker container so it's Ubuntu.

How can I restart shiny to read in the recent changes I made to the shiny-server.conf file?