Add CI rollout baseline

[ohmyfelix]

What changed

• Added a Nexus smoke-test Makefile target that starts the image, waits for port 8081, and checks the HTTP status.
• Split the Docker workflow into Test, Build, and Docs jobs, including Docker Hub description publishing for dockette/nexus.
• Normalized README badges to the Dockette Copybara style and added the standard Maintenance section.

Why

• Establishes the Dockette CI rollout baseline for dockette/nexus while preserving existing build and run commands.