Add CI rollout baseline

[ohmyfelix]

What changed

• Added a focused Docker workflow for this legacy compose stack: compose/script validation, selected local image builds, and Docker Hub README sync on master.
• Added Makefile baseline targets for selected builds, safe compose validation, and compose startup.
• Normalized README badges to the Dockette copybara style and removed the old Gitter badge.
• Added repository-local AGENTS.md guidance plus CLAUDE.md handoff.

Why

• devstack is still an active Docker/Compose image repository, but a full generic image publish matrix would be risky for this legacy stack. This keeps CI scoped to safe compose validation and representative image builds while preserving existing runtime behavior.