Add CI rollout baseline

[ohmyfelix]

What changed

• Add a Test job that builds the local image and runs a non-secret Kumatron HTTP smoke probe.
• Gate the reusable Docker Build job on Test and add Docker Hub description publishing for dockette/kumatron.
• Add a run target, keep S3-backed testing manual, and normalize README badges plus Maintenance guidance.

Why

• Brings dockette/kumatron in line with the Dockette CI rollout baseline while keeping secret-dependent S3 tests out of default CI.

[f3l1x]

Simplify it, oneline, just test if webserver is working