fix: use exec-form selfhost healthcheck

[Frexxty]

Summary

• change the self-host Docker healthcheck to exec form so Docker does not invoke /bin/sh
• keeps the existing Bun-based /api/health probe unchanged

Test Plan

• docker build -f apps/host-selfhost/Dockerfile -t executor-selfhost:healthcheck-fix .
• docker run -d --name selfhost-healthcheck-test -p 4789:4788 -e BETTER_AUTH_SECRET=... -e EXECUTOR_BOOTSTRAP_ADMIN_EMAIL=smoke@example.com -e EXECUTOR_BOOTSTRAP_ADMIN_PASSWORD=... -e EXECUTOR_WEB_BASE_URL=http://localhost:4789 executor-selfhost:healthcheck-fix
• observed Docker health status become healthy
• curl -fsS http://127.0.0.1:4789/api/health returned {"status":"ok"}