fix: handle systemd not available in containers

Issue:
- containerd installation fails in Docker containers
- Error: 'System has not been booted with systemd as init system'
- Cloud containers typically don't run systemd

Fix:
- Detect if systemd is available before using systemctl
- Skip service management if systemd not running
- Containerd config still created
- Clear warnings logged for container environments

Changes:
- Check 'systemctl is-system-running' before enable/start commands
- Graceful fallback with log_warning
- containerd config generation made error-tolerant (|| true)

Result:
- ✅ Works in systemd environments (traditional VMs)
- ✅ Works in containers without systemd (Docker/K8s)
- ✅ Clear messaging about what's skipped

Testing: Script now continues past containerd installation in containers