Migrate teslamate to native Dagger container.py (#333)

## Summary
- Replace legacy Dockerfile with native Dagger `container.py` build
- Two-stage pipeline: Elixir+Node builder, Debian slim runtime
- Uses shared helpers (`clone_from_forge`, `oci_labels`)
- Delete old Dockerfile (pipeline auto-discovers container.py)
- Update build-container-image docs and mark service reviewed

## Test plan
- [x] `dagger call build --src=. --container-name=teslamate` succeeds locally
- [ ] CI container build passes
- [ ] Deploy from branch and verify teslamate starts cleanly

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Reviewed-on: #333

service-versions.yaml

@@ -190,7 +190,7 @@ services:
 
   - name: teslamate
     type: argocd
-    last-reviewed: 2026-03-03
+    last-reviewed: 2026-04-14
     current-version: "v3.0.0"
     upstream-source: https://github.com/teslamate-org/teslamate/releases