Quick Start assumes Docker is installed on Linux #267

Problem

Following the Quick Start on a fresh Ubuntu / WSL environment fails because
Docker is not installed by default.

The guide assumes docker is available, but on Linux this requires
manual installation.

Impact

New users get stuck early with:
docker: command not found

Suggestion

Add a short Linux-specific Docker installation step to the Quick Start,
or clarify that Docker must be installed separately on Linux/WSL.