fix: use runuser for privilege drop and capture container stderr in logs

The container still exited immediately after the su-based fix.  can
interfere with TTY/stdin handling for interactive shells. Switch to
, which is root-only, skips PAM, and preserves file descriptors so
bash stays interactive.

Also improve container failure diagnostics:
-  now combines stdout and stderr
- This helps surface the real reason when a container exits with code 0

Quality gates:
- pytest tests/unit: 219 passed
- ruff: clean on changed files
- mypy: clean on changed files

apps/.pi-map.index.md

@@ -2,7 +2,7 @@
 dir: apps
 
 ## role
-Contains the main deployable application modules that compose the project's executable entry points.
+Contains the main deployable application modules and entry points for the project.
 ## parent
 index: ./.pi-map.index.md
 map: ./.pi-map.md