Help - Newbie Docker

Hi , can someone please help me. I am a newbie. I have just install HASS OS on a PI4. I al trying to follow the install doc but having some issues. Firstly does Docker come inbuilt with HASS OS image ?
How do I navigate around the files . At the ha > prompt , I type ls and get error . My aim is to install HASS properly with Duckdns and start my automation process thereafter. I am not sure what to do next .
HASS is running on the PI4. I can access it from my phone and PC ( Windows ) I have added Samba Share and DuckDns ( with token and domain ) . How do I configure HASS to use DuckDns ? Please help… Thanking you in Advance