Update nav2_demo to match the new build approach

[xfiderek]

See discussion https://github.com/space-ros/space-ros/discussions/140 and issue https://github.com/space-ros/space-ros/issues/107 for more context

TODOs

• move dockerfile to demos repo
• Use a new generate_repos.sh file from updated spaceros dev image and generate pinned .repos file
• Use a separate container moveit2 and nav2 (if not done)

[mkhansenbot]

Great, who's submitting the PR? If no one else is I'll do this one