feat: add --version, a health check that tracks DockTail, and an update notice

[marvinvr]

Makes DockTail easier to operate and support.

Version

• New version package; release builds inject the version there via -ldflags (the cloud module reads the same value).
• The startup log carries it: Starting DockTail version=….
• docktail --version prints it (docker exec docktail /app/docktail --version).

Health check that tracks DockTail

• The running process rewrites a small JSON status file (HEALTH_FILE, default /tmp/docktail-health.json) every 10 s and after every reconcile. docktail health reads it, prints a one-line verdict, and exits 0 or 1. It is now the image's HEALTHCHECK, replacing tailscale serve status. Nothing listens on a port.
• Unhealthy when:
  • the file is stale for 60 s (DockTail is hung or stopped);
  • the tailscaled socket does not accept connections;
  • two reconciles in a row could not read the containers from Docker;
  • no reconcile has finished for three intervals (at least 3 min).
• Per-service reconcile errors (label conflicts, refused services) are reported as healthy, with errors.
• The DockTail Cloud link state (connecting, connected, disconnected, rejected, failed) is reported but never fails the check. A restart triggered by the check (Swarm, autoheal) would drain every service and fix nothing on the cloud side.

Update notice

• About 30 s after startup and then daily, release builds look up the repository's tags on the GitHub API. When a newer stable release exists, they log it once. UPDATE_CHECK=false disables this, and dev builds skip it.

Cloud module

• Records its link state for the health status.
• Reads hello_ack.server_time and warns (at most hourly) when the host clock is more than 60 s off.

Also adds unit tests for semver and the health verdict, e2e assertions for docktail health and --version, and docs in docs/07-reference.md (Version And Updates, Health Check, the new env vars) and docs/06-cloud.md. ENTRYPOINT and packaging are unchanged.

Refs marvinvr/docktail-cloud#81