perf(runner): replace systemd with minimal init for faster boot

[lancy]

Summary

Replace systemd with a minimal init system to reduce guest boot time by 3-5 seconds.

Problem

Current guest image uses systemd as init system:

• systemd has inherent startup overhead
• Even with services stripped, systemd does dependency resolution
• journald, udev stubs, and other subsystems still initialize
• Estimated overhead: 3-5 seconds

Proposed Solution

Replace systemd with a minimal init script that only:

1. Sets up network interface (already configured via kernel args)
2. Starts SSH daemon
3. (Optional) Starts vsock agent

Target Boot Sequence

kernel boot → minimal init → sshd ready

vs current:

kernel boot → systemd → service resolution → sshd ready

Technical Approach

Option A: Custom Shell Init

#!/bin/sh
# /sbin/init
mount -t proc proc /proc
mount -t sysfs sys /sys
mount -t devtmpfs dev /dev

# Network already configured via kernel ip= parameter
/usr/sbin/sshd -D &

# Keep init running
exec /bin/sh

Option B: Busybox Init

Use busybox init with minimal inittab.

Option C: tini/dumb-init

Minimal init designed for containers, adapted for VM.

Key Files

• turbo/scripts/deploy-runner/Dockerfile - Replace systemd
• turbo/scripts/deploy-runner/build-rootfs.sh - Update verification

Expected Impact

• Time saved: ~3-5 seconds per VM startup
• Image size: Smaller (no systemd packages)

Trade-offs

Aspect	systemd	Minimal Init
Boot time	Slower	Faster
Service management	Full	Manual
Debugging	journalctl	Manual logs
Maintenance	Familiar	Custom

Considerations

• Lose systemd service management
• Need custom scripts for any additional services
• Less debugging tooling available
• Consider keeping systemd as fallback option

Alternative: Alpine Linux

Could also consider Alpine Linux base image which uses OpenRC (lighter than systemd) and has much smaller footprint (~100MB vs 2GB).